diff --git a/toolkit/components/extensions/.eslintrc.js b/toolkit/components/extensions/.eslintrc.js
index f62ff67ff06967..777ca9bf56d1ac 100644
--- a/toolkit/components/extensions/.eslintrc.js
+++ b/toolkit/components/extensions/.eslintrc.js
@@ -56,8 +56,22 @@ module.exports = {
       },
     ],
 
-    // No using variables before defined
-    "no-use-before-define": "error",
+    // No using things before they're defined.
+    "no-use-before-define": [
+      "error",
+      {
+        allowNamedExports: true,
+        classes: true,
+        // The next two being false allows idiomatic patterns which are more
+        // type-inference friendly.  Functions are hoisted, so this is safe.
+        functions: false,
+        // This flag is only meaningful for `var` declarations.
+        // When false, it still disallows use-before-define in the same scope.
+        // Since we only allow `var` at the global scope, this is no worse than
+        // how we currently declare an uninitialized `let` at the top of file.
+        variables: false,
+      },
+    ],
 
     // Disallow using variables outside the blocks they are defined (especially
     // since only let and const are used, see "no-var").
diff --git a/toolkit/components/extensions/ConduitsParent.sys.mjs b/toolkit/components/extensions/ConduitsParent.sys.mjs
index e0debfd1876a04..3342315323823c 100644
--- a/toolkit/components/extensions/ConduitsParent.sys.mjs
+++ b/toolkit/components/extensions/ConduitsParent.sys.mjs
@@ -452,13 +452,13 @@ export class ConduitsParent extends JSWindowActorParent {
       return Hub.recvConduitOpened(arg, this);
     }
 
-    sender = Hub.remotes.get(sender);
-    if (!sender || sender.actor !== this) {
+    let remote = Hub.remotes.get(sender);
+    if (!remote || remote.actor !== this) {
       throw new Error(`Unknown sender or wrong actor for recv${name}`);
     }
 
     if (name === "ConduitClosed") {
-      return Hub.recvConduitClosed(sender);
+      return Hub.recvConduitClosed(remote);
     }
 
     let conduit = Hub.byMethod.get(name);
@@ -466,7 +466,7 @@ export class ConduitsParent extends JSWindowActorParent {
       throw new Error(`Parent conduit for recv${name} not found`);
     }
 
-    return conduit._recv(name, arg, { actor: this, query, sender });
+    return conduit._recv(name, arg, { actor: this, query, sender: remote });
   }
 
   /**
diff --git a/toolkit/components/extensions/Extension.sys.mjs b/toolkit/components/extensions/Extension.sys.mjs
index 164c0bfc461d11..7aafa867be080c 100644
--- a/toolkit/components/extensions/Extension.sys.mjs
+++ b/toolkit/components/extensions/Extension.sys.mjs
@@ -172,7 +172,7 @@ export { Management };
 
 const { getUniqueId, promiseTimeout } = ExtensionUtils;
 
-const { EventEmitter, updateAllowedOrigins } = ExtensionCommon;
+const { EventEmitter, redefineGetter, updateAllowedOrigins } = ExtensionCommon;
 
 ChromeUtils.defineLazyGetter(
   lazy,
@@ -869,6 +869,19 @@ const manifestTypes = new Map([
  * `loadManifest` has been called, and completed.
  */
 export class ExtensionData {
+  /**
+   * Note: These fields are only available and meant to be used on Extension
+   * instances, declared here because methods from this class reference them.
+   */
+  /** @type {object} TODO: move to the Extension class, bug 1871094. */
+  addonData;
+  /** @type {nsIURI} */
+  baseURI;
+  /** @type {nsIPrincipal} */
+  principal;
+  /** @type {boolean} */
+  temporarilyInstalled;
+
   constructor(rootURI, isPrivileged = false) {
     this.rootURI = rootURI;
     this.resourceURL = rootURI.spec;
@@ -2676,7 +2689,7 @@ class BootstrapScope {
     // APP_STARTED.  In some situations, such as background and
     // persisted listeners, we also need to know that the addon
     // was updated.
-    this.updateReason = this.BOOTSTRAP_REASON_TO_STRING_MAP[reason];
+    this.updateReason = BootstrapScope.BOOTSTRAP_REASON_MAP[reason];
     // Retain any previously granted permissions that may have migrated
     // into the optional list.
     if (data.oldPermissions) {
@@ -2703,7 +2716,7 @@ class BootstrapScope {
     // eslint-disable-next-line no-use-before-define
     this.extension = new Extension(
       data,
-      this.BOOTSTRAP_REASON_TO_STRING_MAP[reason],
+      BootstrapScope.BOOTSTRAP_REASON_MAP[reason],
       this.updateReason
     );
     return this.extension.startup();
@@ -2711,31 +2724,27 @@ class BootstrapScope {
 
   async shutdown(data, reason) {
     let result = await this.extension.shutdown(
-      this.BOOTSTRAP_REASON_TO_STRING_MAP[reason]
+      BootstrapScope.BOOTSTRAP_REASON_MAP[reason]
     );
     this.extension = null;
     return result;
   }
-}
 
-ChromeUtils.defineLazyGetter(
-  BootstrapScope.prototype,
-  "BOOTSTRAP_REASON_TO_STRING_MAP",
-  () => {
-    const { BOOTSTRAP_REASONS } = lazy.AddonManagerPrivate;
-
-    return Object.freeze({
-      [BOOTSTRAP_REASONS.APP_STARTUP]: "APP_STARTUP",
-      [BOOTSTRAP_REASONS.APP_SHUTDOWN]: "APP_SHUTDOWN",
-      [BOOTSTRAP_REASONS.ADDON_ENABLE]: "ADDON_ENABLE",
-      [BOOTSTRAP_REASONS.ADDON_DISABLE]: "ADDON_DISABLE",
-      [BOOTSTRAP_REASONS.ADDON_INSTALL]: "ADDON_INSTALL",
-      [BOOTSTRAP_REASONS.ADDON_UNINSTALL]: "ADDON_UNINSTALL",
-      [BOOTSTRAP_REASONS.ADDON_UPGRADE]: "ADDON_UPGRADE",
-      [BOOTSTRAP_REASONS.ADDON_DOWNGRADE]: "ADDON_DOWNGRADE",
+  static get BOOTSTRAP_REASON_MAP() {
+    const BR = lazy.AddonManagerPrivate.BOOTSTRAP_REASONS;
+    const value = Object.freeze({
+      [BR.APP_STARTUP]: "APP_STARTUP",
+      [BR.APP_SHUTDOWN]: "APP_SHUTDOWN",
+      [BR.ADDON_ENABLE]: "ADDON_ENABLE",
+      [BR.ADDON_DISABLE]: "ADDON_DISABLE",
+      [BR.ADDON_INSTALL]: "ADDON_INSTALL",
+      [BR.ADDON_UNINSTALL]: "ADDON_UNINSTALL",
+      [BR.ADDON_UPGRADE]: "ADDON_UPGRADE",
+      [BR.ADDON_DOWNGRADE]: "ADDON_DOWNGRADE",
     });
+    return redefineGetter(this, "BOOTSTRAP_REASON_TO_STRING_MAP", value);
   }
-);
+}
 
 class DictionaryBootstrapScope extends BootstrapScope {
   install(data, reason) {}
@@ -2744,11 +2753,11 @@ class DictionaryBootstrapScope extends BootstrapScope {
   startup(data, reason) {
     // eslint-disable-next-line no-use-before-define
     this.dictionary = new Dictionary(data);
-    return this.dictionary.startup(this.BOOTSTRAP_REASON_TO_STRING_MAP[reason]);
+    return this.dictionary.startup(BootstrapScope.BOOTSTRAP_REASON_MAP[reason]);
   }
 
-  shutdown(data, reason) {
-    this.dictionary.shutdown(this.BOOTSTRAP_REASON_TO_STRING_MAP[reason]);
+  async shutdown(data, reason) {
+    this.dictionary.shutdown(BootstrapScope.BOOTSTRAP_REASON_MAP[reason]);
     this.dictionary = null;
   }
 }
@@ -2756,16 +2765,16 @@ class DictionaryBootstrapScope extends BootstrapScope {
 class LangpackBootstrapScope extends BootstrapScope {
   install(data, reason) {}
   uninstall(data, reason) {}
-  update(data, reason) {}
+  async update(data, reason) {}
 
   startup(data, reason) {
     // eslint-disable-next-line no-use-before-define
     this.langpack = new Langpack(data);
-    return this.langpack.startup(this.BOOTSTRAP_REASON_TO_STRING_MAP[reason]);
+    return this.langpack.startup(BootstrapScope.BOOTSTRAP_REASON_MAP[reason]);
   }
 
-  shutdown(data, reason) {
-    this.langpack.shutdown(this.BOOTSTRAP_REASON_TO_STRING_MAP[reason]);
+  async shutdown(data, reason) {
+    this.langpack.shutdown(BootstrapScope.BOOTSTRAP_REASON_MAP[reason]);
     this.langpack = null;
   }
 }
@@ -2779,12 +2788,12 @@ class SitePermissionBootstrapScope extends BootstrapScope {
     // eslint-disable-next-line no-use-before-define
     this.sitepermission = new SitePermission(data);
     return this.sitepermission.startup(
-      this.BOOTSTRAP_REASON_TO_STRING_MAP[reason]
+      BootstrapScope.BOOTSTRAP_REASON_MAP[reason]
     );
   }
 
-  shutdown(data, reason) {
-    this.sitepermission.shutdown(this.BOOTSTRAP_REASON_TO_STRING_MAP[reason]);
+  async shutdown(data, reason) {
+    this.sitepermission.shutdown(BootstrapScope.BOOTSTRAP_REASON_MAP[reason]);
     this.sitepermission = null;
   }
 }
@@ -2800,6 +2809,9 @@ let pendingExtensions = new Map();
  * @augments ExtensionData
  */
 export class Extension extends ExtensionData {
+  /** @type {Map<string, Map<string, any>>} */
+  persistentListeners;
+
   constructor(addonData, startupReason, updateReason) {
     super(addonData.resourceURI, addonData.isPrivileged);
 
@@ -2832,6 +2844,7 @@ export class Extension extends ExtensionData {
     this.startupData = addonData.startupData || {};
     this.startupReason = startupReason;
     this.updateReason = updateReason;
+    this.temporarilyInstalled = !!addonData.temporarilyInstalled;
 
     if (
       updateReason ||
@@ -3077,10 +3090,6 @@ export class Extension extends ExtensionData {
     return [this.id, this.version, Services.locale.appLocaleAsBCP47];
   }
 
-  get temporarilyInstalled() {
-    return !!this.addonData.temporarilyInstalled;
-  }
-
   saveStartupData() {
     if (this.dontSaveStartupData) {
       return;
diff --git a/toolkit/components/extensions/ExtensionActions.sys.mjs b/toolkit/components/extensions/ExtensionActions.sys.mjs
index 33a5d10072190b..8e8cf3abd28544 100644
--- a/toolkit/components/extensions/ExtensionActions.sys.mjs
+++ b/toolkit/components/extensions/ExtensionActions.sys.mjs
@@ -43,6 +43,7 @@ class PanelActionBase {
       enabled: true,
       title: options.default_title || extension.name,
       popup: options.default_popup || "",
+      icon: null,
     };
     this.globals = Object.create(this.defaults);
 
diff --git a/toolkit/components/extensions/ExtensionChild.sys.mjs b/toolkit/components/extensions/ExtensionChild.sys.mjs
index 91bc92bed60f2a..a06ac8024f11f9 100644
--- a/toolkit/components/extensions/ExtensionChild.sys.mjs
+++ b/toolkit/components/extensions/ExtensionChild.sys.mjs
@@ -39,7 +39,7 @@ import { ExtensionUtils } from "resource://gre/modules/ExtensionUtils.sys.mjs";
 const { DefaultMap, ExtensionError, LimitedSet, getUniqueId } = ExtensionUtils;
 
 const {
-  defineLazyGetter,
+  redefineGetter,
   EventEmitter,
   EventManager,
   LocalAPIImplementation,
@@ -299,12 +299,13 @@ class Port {
     }
     throw new this.context.Error("Attempt to postMessage on disconnected port");
   }
-}
 
-defineLazyGetter(Port.prototype, "api", function () {
-  let api = this.getAPI();
-  return Cu.cloneInto(api, this.context.cloneScope, { cloneFunctions: true });
-});
+  get api() {
+    const scope = this.context.cloneScope;
+    const value = Cu.cloneInto(this.getAPI(), scope, { cloneFunctions: true });
+    return redefineGetter(this, "api", value);
+  }
+}
 
 /**
  * Each extension context gets its own Messenger object. It handles the
@@ -522,7 +523,7 @@ class BrowserExtensionContent extends EventEmitter {
 
   emit(event, ...args) {
     Services.cpmm.sendAsyncMessage(this.MESSAGE_EMIT_EVENT, { event, args });
-    super.emit(event, ...args);
+    return super.emit(event, ...args);
   }
 
   // TODO(Bug 1768471): consider folding this back into emit if we will change it to
@@ -914,10 +915,10 @@ class ChildAPIManager {
    *   hasListener methods. See SchemaAPIInterface for documentation.
    */
   getParentEvent(path) {
-    path = path.split(".");
+    let parts = path.split(".");
 
-    let name = path.pop();
-    let namespace = path.join(".");
+    let name = parts.pop();
+    let namespace = parts.join(".");
 
     let impl = new ProxyAPIImplementation(namespace, name, this, true);
     return {
diff --git a/toolkit/components/extensions/ExtensionCommon.sys.mjs b/toolkit/components/extensions/ExtensionCommon.sys.mjs
index 517fedf0e09c09..e4cd09c0c884ff 100644
--- a/toolkit/components/extensions/ExtensionCommon.sys.mjs
+++ b/toolkit/components/extensions/ExtensionCommon.sys.mjs
@@ -54,8 +54,6 @@ function getConsole() {
   });
 }
 
-export var ExtensionCommon;
-
 // Run a function and report exceptions.
 function runSafeSyncWithoutClone(f, ...args) {
   try {
@@ -122,6 +120,8 @@ function withHandlingUserInput(window, callable) {
  * prototype will be invoked separately for each object instance that
  * it's accessed on.
  *
+ * Note: for better type inference, prefer redefineGetter() below.
+ *
  * @param {object} object
  *        The prototype object on which to define the getter.
  * @param {string | symbol} prop
@@ -131,30 +131,39 @@ function withHandlingUserInput(window, callable) {
  *        value.
  */
 function defineLazyGetter(object, prop, getter) {
-  let redefine = (obj, value) => {
-    Object.defineProperty(obj, prop, {
-      enumerable: true,
-      configurable: true,
-      writable: true,
-      value,
-    });
-    return value;
-  };
-
   Object.defineProperty(object, prop, {
     enumerable: true,
     configurable: true,
-
     get() {
-      return redefine(this, getter.call(this));
+      return redefineGetter(this, prop, getter.call(this), true);
     },
-
     set(value) {
-      redefine(this, value);
+      redefineGetter(this, prop, value, true);
     },
   });
 }
 
+/**
+ * A more type-inference friendly version of defineLazyGetter() above.
+ * Call it from a real getter (and setter) for your class or object.
+ * On first run, it will redefine the property with the final value.
+ *
+ * @template Value
+ * @param {object} object
+ * @param {string | symbol} key
+ * @param {Value} value
+ * @returns {Value}
+ */
+function redefineGetter(object, key, value, writable = false) {
+  Object.defineProperty(object, key, {
+    enumerable: true,
+    configurable: true,
+    writable,
+    value,
+  });
+  return value;
+}
+
 function checkLoadURI(uri, principal, options) {
   let ssm = Services.scriptSecurityManager;
 
@@ -292,11 +301,11 @@ class EventEmitter {
    *        The listener to call when events are emitted.
    */
   once(event, listener) {
-    let wrapper = (...args) => {
+    let wrapper = (event, ...args) => {
       this.off(event, wrapper);
       this[ONCE_MAP].delete(listener);
 
-      return listener(...args);
+      return listener(event, ...args);
     };
     this[ONCE_MAP].set(listener, wrapper);
 
@@ -360,11 +369,28 @@ class ExtensionAPI extends EventEmitter {
 
   destroy() {}
 
-  onManifestEntry(entry) {}
+  /** @param {string} entryName */
+  onManifestEntry(entryName) {}
 
+  /** @param {boolean} isAppShutdown */
+  onShutdown(isAppShutdown) {}
+
+  /** @param {BaseContext} context */
   getAPI(context) {
     throw new Error("Not Implemented");
   }
+
+  /** @param {string} id */
+  static onDisable(id) {}
+
+  /** @param {string} id */
+  static onUninstall(id) {}
+
+  /**
+   * @param {string} id
+   * @param {Record<string, JSONValue>} manifest
+   */
+  static onUpdate(id, manifest) {}
 }
 
 /**
@@ -374,6 +400,9 @@ class ExtensionAPI extends EventEmitter {
  * this.apiNamespace = class extends ExtensionAPIPersistent {};
  */
 class ExtensionAPIPersistent extends ExtensionAPI {
+  /** @type {Record<string, callback>} */
+  PERSISTENT_EVENTS;
+
   /**
    * Check for event entry.
    *
@@ -440,6 +469,11 @@ class ExtensionAPIPersistent extends ExtensionAPI {
  * @abstract
  */
 class BaseContext {
+  /** @type {boolean} */
+  isTopContext;
+  /** @type {string} */
+  viewType;
+
   constructor(envType, extension) {
     this.envType = envType;
     this.onClose = new Set();
@@ -1895,6 +1929,7 @@ class LazyAPIManager extends SchemaAPIManager {
   constructor(processType, moduleData, schemaURLs) {
     super(processType);
 
+    /** @type {Promise | boolean} */
     this.initialized = false;
 
     this.initModuleData(moduleData);
@@ -1979,7 +2014,7 @@ defineLazyGetter(MultiAPIManager.prototype, "schema", function () {
   return new lazy.SchemaRoot(bases, new Map());
 });
 
-function LocaleData(data) {
+export function LocaleData(data) {
   this.defaultLocale = data.defaultLocale;
   this.selectedLocale = data.selectedLocale;
   this.locales = data.locales || new Map();
@@ -2186,15 +2221,13 @@ LocaleData.prototype = {
   get uiLocale() {
     return Services.locale.appLocaleAsBCP47;
   },
-};
 
-defineLazyGetter(LocaleData.prototype, "availableLocales", function () {
-  return new Set(
-    [this.BUILTIN, this.selectedLocale, this.defaultLocale].filter(locale =>
-      this.messages.has(locale)
-    )
-  );
-});
+  get availableLocales() {
+    const locales = [this.BUILTIN, this.selectedLocale, this.defaultLocale];
+    const value = new Set(locales.filter(locale => this.messages.has(locale)));
+    return redefineGetter(this, "availableLocales", value);
+  },
+};
 
 /**
  * This is a generic class for managing event listeners.
@@ -3012,7 +3045,7 @@ function updateAllowedOrigins(policy, origins, isAdd) {
   policy.allowedOrigins = new MatchPatternSet(Array.from(patternMap.values()));
 }
 
-ExtensionCommon = {
+export var ExtensionCommon = {
   BaseContext,
   CanOfAPIs,
   EventManager,
@@ -3028,6 +3061,7 @@ ExtensionCommon = {
   checkLoadURI,
   checkLoadURL,
   defineLazyGetter,
+  redefineGetter,
   getConsole,
   ignoreEvent,
   instanceOf,
diff --git a/toolkit/components/extensions/ExtensionContent.sys.mjs b/toolkit/components/extensions/ExtensionContent.sys.mjs
index 9b67de4360a222..131d555bf0adf1 100644
--- a/toolkit/components/extensions/ExtensionContent.sys.mjs
+++ b/toolkit/components/extensions/ExtensionContent.sys.mjs
@@ -59,6 +59,7 @@ const {
   CanOfAPIs,
   SchemaAPIManager,
   defineLazyGetter,
+  redefineGetter,
   runSafeSyncWithoutClone,
 } = ExtensionCommon;
 
@@ -142,7 +143,7 @@ class CacheMap extends DefaultMap {
       super.get(url).timer.cancel();
     }
 
-    super.delete(url);
+    return super.delete(url);
   }
 
   clear(timeout = SCRIPT_CLEAR_TIMEOUT_MS) {
@@ -160,16 +161,17 @@ class CacheMap extends DefaultMap {
 
 class ScriptCache extends CacheMap {
   constructor(options, extension) {
-    super(SCRIPT_EXPIRY_TIMEOUT_MS, null, extension);
-    this.options = options;
-  }
-
-  defaultConstructor(url) {
-    let promise = ChromeUtils.compileScript(url, this.options);
-    promise.then(script => {
-      promise.script = script;
-    });
-    return promise;
+    super(
+      SCRIPT_EXPIRY_TIMEOUT_MS,
+      url => {
+        let promise = ChromeUtils.compileScript(url, options);
+        promise.then(script => {
+          promise.script = script;
+        });
+        return promise;
+      },
+      extension
+    );
   }
 }
 
@@ -207,7 +209,7 @@ class BaseCSSCache extends CacheMap {
       }
     }
 
-    super.delete(key);
+    return super.delete(key);
   }
 }
 
@@ -907,13 +909,6 @@ class ContentScriptContextChild extends BaseContext {
 
     this.url = contentWindow.location.href;
 
-    defineLazyGetter(this, "chromeObj", () => {
-      let chromeObj = Cu.createObjectIn(this.sandbox);
-
-      this.childManager.inject(chromeObj);
-      return chromeObj;
-    });
-
     lazy.Schemas.exportLazyGetter(
       this.sandbox,
       "browser",
@@ -993,31 +988,28 @@ class ContentScriptContextChild extends BaseContext {
 
     this.sandbox = null;
   }
-}
-
-defineLazyGetter(ContentScriptContextChild.prototype, "messenger", function () {
-  return new Messenger(this);
-});
 
-defineLazyGetter(
-  ContentScriptContextChild.prototype,
-  "childManager",
-  function () {
+  get childManager() {
     apiManager.lazyInit();
-
-    let localApis = {};
-    let can = new CanOfAPIs(this, apiManager, localApis);
-
+    let can = new CanOfAPIs(this, apiManager, {});
     let childManager = new ChildAPIManager(this, this.messageManager, can, {
       envType: "content_parent",
       url: this.url,
     });
-
     this.callOnClose(childManager);
+    return redefineGetter(this, "childManager", childManager);
+  }
 
-    return childManager;
+  get chromeObj() {
+    let chromeObj = Cu.createObjectIn(this.sandbox);
+    this.childManager.inject(chromeObj);
+    return redefineGetter(this, "chromeObj", chromeObj);
   }
-);
+
+  get messenger() {
+    return redefineGetter(this, "messenger", new Messenger(this));
+  }
+}
 
 // Responsible for creating ExtensionContexts and injecting content
 // scripts into them when new documents are created.
diff --git a/toolkit/components/extensions/ExtensionDNR.sys.mjs b/toolkit/components/extensions/ExtensionDNR.sys.mjs
index b6d1f68f4659a9..2757333b3b419b 100644
--- a/toolkit/components/extensions/ExtensionDNR.sys.mjs
+++ b/toolkit/components/extensions/ExtensionDNR.sys.mjs
@@ -145,7 +145,7 @@ class RuleCondition {
   }
 }
 
-class Rule {
+export class Rule {
   constructor(rule) {
     this.id = rule.id;
     this.priority = rule.priority;
@@ -633,6 +633,24 @@ class ModifyHeadersBase {
     this.channel = channel;
   }
 
+  /**
+   * @param {MatchedRule} matchedRule
+   * @returns {object[]}
+   */
+  headerActionsFor(matchedRule) {
+    throw new Error("Not implemented.");
+  }
+
+  /**
+   * @param {MatchedRule} matchedrule
+   * @param {string} name
+   * @param {string} value
+   * @param {boolean} merge
+   */
+  setHeaderImpl(matchedrule, name, value, merge) {
+    throw new Error("Not implemented.");
+  }
+
   /** @param {MatchedRule[]} matchedRules */
   applyModifyHeaders(matchedRules) {
     for (const matchedRule of matchedRules) {
@@ -1190,7 +1208,7 @@ class RuleValidator {
   }
 }
 
-class RuleQuotaCounter {
+export class RuleQuotaCounter {
   constructor(isStaticRulesets) {
     this.isStaticRulesets = isStaticRulesets;
     this.ruleLimitName = isStaticRulesets
diff --git a/toolkit/components/extensions/ExtensionDNRStore.sys.mjs b/toolkit/components/extensions/ExtensionDNRStore.sys.mjs
index 2e8777cc4281eb..0dd6fad75a6cb3 100644
--- a/toolkit/components/extensions/ExtensionDNRStore.sys.mjs
+++ b/toolkit/components/extensions/ExtensionDNRStore.sys.mjs
@@ -148,7 +148,7 @@ class StoreData {
     if (!(extension instanceof lazy.Extension)) {
       throw new Error("Missing mandatory extension parameter");
     }
-    this.schemaVersion = schemaVersion || this.constructor.VERSION;
+    this.schemaVersion = schemaVersion || StoreData.VERSION;
     this.extVersion = extVersion ?? extension.version;
     this.#extUUID = extension.uuid;
     // Used to skip storing the data in the startupCache or storing the lastUpdateTag in
@@ -220,8 +220,8 @@ class StoreData {
   }
 
   #updateRulesets({
-    staticRulesets,
-    dynamicRuleset,
+    staticRulesets = null,
+    dynamicRuleset = null,
     lastUpdateTag = Services.uuid.generateUUID().toString(),
   } = {}) {
     if (staticRulesets) {
@@ -667,11 +667,7 @@ class RulesetsStore {
    * @returns {StoreData}
    */
   #getDefaults(extension) {
-    return new StoreData(extension, {
-      extUUID: extension.uuid,
-      extVersion: extension.version,
-      temporarilyInstalled: extension.temporarilyInstalled,
-    });
+    return new StoreData(extension, { extVersion: extension.version });
   }
 
   /**
diff --git a/toolkit/components/extensions/ExtensionPageChild.sys.mjs b/toolkit/components/extensions/ExtensionPageChild.sys.mjs
index 7c051c511e9252..d84459f1edb9a4 100644
--- a/toolkit/components/extensions/ExtensionPageChild.sys.mjs
+++ b/toolkit/components/extensions/ExtensionPageChild.sys.mjs
@@ -29,13 +29,11 @@ import { ExtensionUtils } from "resource://gre/modules/ExtensionUtils.sys.mjs";
 
 const { getInnerWindowID, promiseEvent } = ExtensionUtils;
 
-const { BaseContext, CanOfAPIs, SchemaAPIManager, defineLazyGetter } =
+const { BaseContext, CanOfAPIs, SchemaAPIManager, redefineGetter } =
   ExtensionCommon;
 
 const { ChildAPIManager, Messenger } = ExtensionChild;
 
-export var ExtensionPageChild;
-
 const initializeBackgroundPage = context => {
   // Override the `alert()` method inside background windows;
   // we alias it to console.log().
@@ -185,7 +183,7 @@ export function getContextChildManagerGetter(
   };
 }
 
-class ExtensionBaseContextChild extends BaseContext {
+export class ExtensionBaseContextChild extends BaseContext {
   /**
    * This ExtensionBaseContextChild represents an addon execution environment
    * that is running in an addon or devtools child process.
@@ -220,12 +218,6 @@ class ExtensionBaseContextChild extends BaseContext {
       });
     }
 
-    defineLazyGetter(this, "browserObj", () => {
-      let browserObj = Cu.createObjectIn(contentWindow);
-      this.childManager.inject(browserObj);
-      return browserObj;
-    });
-
     lazy.Schemas.exportLazyGetter(contentWindow, "browser", () => {
       return this.browserObj;
     });
@@ -245,6 +237,12 @@ class ExtensionBaseContextChild extends BaseContext {
     });
   }
 
+  get browserObj() {
+    const browserObj = Cu.createObjectIn(this.contentWindow);
+    this.childManager.inject(browserObj);
+    return redefineGetter(this, "browserObj", browserObj);
+  }
+
   logActivity(type, name, data) {
     ExtensionActivityLogChild.log(this, type, name, data);
   }
@@ -283,11 +281,16 @@ class ExtensionBaseContextChild extends BaseContext {
 
     super.unload();
   }
-}
 
-defineLazyGetter(ExtensionBaseContextChild.prototype, "messenger", function () {
-  return new Messenger(this);
-});
+  get messenger() {
+    return redefineGetter(this, "messenger", new Messenger(this));
+  }
+
+  /** @type {ReturnType<ReturnType<getContextChildManagerGetter>>} */
+  get childManager() {
+    throw new Error("childManager getter must be overridden");
+  }
+}
 
 class ExtensionPageContextChild extends ExtensionBaseContextChild {
   /**
@@ -322,15 +325,16 @@ class ExtensionPageContextChild extends ExtensionBaseContextChild {
     super.unload();
     this.extension.views.delete(this);
   }
-}
 
-defineLazyGetter(
-  ExtensionPageContextChild.prototype,
-  "childManager",
-  getContextChildManagerGetter({ envType: "addon_parent" })
-);
+  get childManager() {
+    const childManager = getContextChildManagerGetter({
+      envType: "addon_parent",
+    }).call(this);
+    return redefineGetter(this, "childManager", childManager);
+  }
+}
 
-class DevToolsContextChild extends ExtensionBaseContextChild {
+export class DevToolsContextChild extends ExtensionBaseContextChild {
   /**
    * This DevToolsContextChild represents a devtools-related addon execution
    * environment that has access to the devtools API namespace and to the same subset
@@ -361,15 +365,16 @@ class DevToolsContextChild extends ExtensionBaseContextChild {
     super.unload();
     this.extension.devtoolsViews.delete(this);
   }
-}
 
-defineLazyGetter(
-  DevToolsContextChild.prototype,
-  "childManager",
-  getContextChildManagerGetter({ envType: "devtools_parent" })
-);
+  get childManager() {
+    const childManager = getContextChildManagerGetter({
+      envType: "devtools_parent",
+    }).call(this);
+    return redefineGetter(this, "childManager", childManager);
+  }
+}
 
-ExtensionPageChild = {
+export var ExtensionPageChild = {
   initialized: false,
 
   // Map<innerWindowId, ExtensionPageContextChild>
diff --git a/toolkit/components/extensions/ExtensionParent.sys.mjs b/toolkit/components/extensions/ExtensionParent.sys.mjs
index 1028ac62124159..da90a62954da75 100644
--- a/toolkit/components/extensions/ExtensionParent.sys.mjs
+++ b/toolkit/components/extensions/ExtensionParent.sys.mjs
@@ -46,7 +46,7 @@ const DUMMY_PAGE_URI = Services.io.newURI(
   "chrome://extensions/content/dummy.xhtml"
 );
 
-var { BaseContext, CanOfAPIs, SchemaAPIManager, SpreadArgs, defineLazyGetter } =
+var { BaseContext, CanOfAPIs, SchemaAPIManager, SpreadArgs, redefineGetter } =
   ExtensionCommon;
 
 var {
@@ -72,7 +72,6 @@ schemaURLs.add("chrome://extensions/content/schemas/experiments.json");
 
 let GlobalManager;
 let ParentAPIManager;
-let StartupCache;
 
 function verifyActorForContext(actor, context) {
   if (JSWindowActorParent.isInstance(actor)) {
@@ -664,26 +663,25 @@ class ProxyContextParent extends BaseContext {
     super.unload();
     apiManager.emit("proxy-context-unload", this);
   }
-}
 
-defineLazyGetter(ProxyContextParent.prototype, "apiCan", function () {
-  let obj = {};
-  let can = new CanOfAPIs(this, this.extension.apiManager, obj);
-  return can;
-});
+  get apiCan() {
+    const apiCan = new CanOfAPIs(this, this.extension.apiManager, {});
+    return redefineGetter(this, "apiCan", apiCan);
+  }
 
-defineLazyGetter(ProxyContextParent.prototype, "apiObj", function () {
-  return this.apiCan.root;
-});
+  get apiObj() {
+    return redefineGetter(this, "apiObj", this.apiCan.root);
+  }
 
-defineLazyGetter(ProxyContextParent.prototype, "sandbox", function () {
-  // NOTE: the required Blob and URL globals are used in the ext-registerContentScript.js
-  // API module to convert JS and CSS data into blob URLs.
-  return Cu.Sandbox(this.principal, {
-    sandboxName: this.uri.spec,
-    wantGlobalProperties: ["Blob", "URL"],
-  });
-});
+  get sandbox() {
+    // Note: Blob and URL globals are used in ext-contentScripts.js.
+    const sandbox = Cu.Sandbox(this.principal, {
+      sandboxName: this.uri.spec,
+      wantGlobalProperties: ["Blob", "URL"],
+    });
+    return redefineGetter(this, "sandbox", sandbox);
+  }
+}
 
 /**
  * The parent side of proxied API context for extension content script
@@ -728,11 +726,6 @@ class ExtensionPageContextParent extends ProxyContextParent {
     }
   }
 
-  onBrowserChange(browser) {
-    super.onBrowserChange(browser);
-    this.xulBrowser = browser;
-  }
-
   unload() {
     super.unload();
     this.extension.views.delete(this);
@@ -1404,7 +1397,7 @@ class HiddenXULWindow {
       }
     }
 
-    let awaitFrameLoader = Promise.resolve();
+    let awaitFrameLoader;
 
     if (browser.getAttribute("remote") === "true") {
       awaitFrameLoader = promiseEvent(browser, "XULFrameLoaderCreated");
@@ -2055,20 +2048,76 @@ let IconDetails = {
   },
 };
 
+class CacheStore {
+  constructor(storeName) {
+    this.storeName = storeName;
+  }
+
+  async getStore(path = null) {
+    let data = await StartupCache.dataPromise;
+
+    let store = data.get(this.storeName);
+    if (!store) {
+      store = new Map();
+      data.set(this.storeName, store);
+    }
+
+    let key = path;
+    if (Array.isArray(path)) {
+      for (let elem of path.slice(0, -1)) {
+        let next = store.get(elem);
+        if (!next) {
+          next = new Map();
+          store.set(elem, next);
+        }
+        store = next;
+      }
+      key = path[path.length - 1];
+    }
+
+    return [store, key];
+  }
+
+  async get(path, createFunc) {
+    let [store, key] = await this.getStore(path);
+
+    let result = store.get(key);
+
+    if (result === undefined) {
+      result = await createFunc(path);
+      store.set(key, result);
+      StartupCache.save();
+    }
+
+    return result;
+  }
+
+  async set(path, value) {
+    let [store, key] = await this.getStore(path);
+
+    store.set(key, value);
+    StartupCache.save();
+  }
+
+  async getAll() {
+    let [store] = await this.getStore();
+
+    return new Map(store);
+  }
+
+  async delete(path) {
+    let [store, key] = await this.getStore(path);
+
+    if (store.delete(key)) {
+      StartupCache.save();
+    }
+  }
+}
+
 // A cache to support faster initialization of extensions at browser startup.
 // All cached data is removed when the browser is updated.
 // Extension-specific data is removed when the add-on is updated.
-StartupCache = {
-  STORE_NAMES: Object.freeze([
-    "general",
-    "locales",
-    "manifests",
-    "other",
-    "permissions",
-    "schemas",
-    "menus",
-  ]),
-
+var StartupCache = {
   _ensureDirectoryPromise: null,
   _saveTask: null,
 
@@ -2179,80 +2228,18 @@ StartupCache = {
   delete(extension, path) {
     return this.general.delete([extension.id, extension.version, ...path]);
   },
+
+  general: new CacheStore("general"),
+  locales: new CacheStore("locales"),
+  manifests: new CacheStore("manifests"),
+  other: new CacheStore("other"),
+  permissions: new CacheStore("permissions"),
+  schemas: new CacheStore("schemas"),
+  menus: new CacheStore("menus"),
 };
 
 Services.obs.addObserver(StartupCache, "startupcache-invalidate");
 
-class CacheStore {
-  constructor(storeName) {
-    this.storeName = storeName;
-  }
-
-  async getStore(path = null) {
-    let data = await StartupCache.dataPromise;
-
-    let store = data.get(this.storeName);
-    if (!store) {
-      store = new Map();
-      data.set(this.storeName, store);
-    }
-
-    let key = path;
-    if (Array.isArray(path)) {
-      for (let elem of path.slice(0, -1)) {
-        let next = store.get(elem);
-        if (!next) {
-          next = new Map();
-          store.set(elem, next);
-        }
-        store = next;
-      }
-      key = path[path.length - 1];
-    }
-
-    return [store, key];
-  }
-
-  async get(path, createFunc) {
-    let [store, key] = await this.getStore(path);
-
-    let result = store.get(key);
-
-    if (result === undefined) {
-      result = await createFunc(path);
-      store.set(key, result);
-      StartupCache.save();
-    }
-
-    return result;
-  }
-
-  async set(path, value) {
-    let [store, key] = await this.getStore(path);
-
-    store.set(key, value);
-    StartupCache.save();
-  }
-
-  async getAll() {
-    let [store] = await this.getStore();
-
-    return new Map(store);
-  }
-
-  async delete(path) {
-    let [store, key] = await this.getStore(path);
-
-    if (store.delete(key)) {
-      StartupCache.save();
-    }
-  }
-}
-
-for (let name of StartupCache.STORE_NAMES) {
-  StartupCache[name] = new CacheStore(name);
-}
-
 export var ExtensionParent = {
   GlobalManager,
   HiddenExtensionPage,
diff --git a/toolkit/components/extensions/ExtensionShortcuts.sys.mjs b/toolkit/components/extensions/ExtensionShortcuts.sys.mjs
index 799cf516b8c32b..7b30fa2cdfa734 100644
--- a/toolkit/components/extensions/ExtensionShortcuts.sys.mjs
+++ b/toolkit/components/extensions/ExtensionShortcuts.sys.mjs
@@ -108,7 +108,7 @@ export class ExtensionShortcutKeyMap extends DefaultMap {
   // Class internals.
 
   constructor() {
-    super();
+    super(() => new Set());
 
     // Overridden in some unit test to make it easier to cover some
     // platform specific behaviors (in particular the platform specific.
@@ -116,10 +116,6 @@ export class ExtensionShortcutKeyMap extends DefaultMap {
     this._os = lazy.ExtensionParent.PlatformInfo.os;
   }
 
-  defaultConstructor() {
-    return new Set();
-  }
-
   getPlatformShortcutString(shortcutString) {
     if (this._os == "mac") {
       // when running on macos, make sure to also track in the shortcutKeyMap
@@ -150,7 +146,7 @@ export class ExtensionShortcutKeyMap extends DefaultMap {
 
   delete(shortcutString) {
     const platformShortcut = this.getPlatformShortcutString(shortcutString);
-    super.delete(platformShortcut);
+    return super.delete(platformShortcut);
   }
 }
 
@@ -397,7 +393,7 @@ export class ExtensionShortcuts {
       this.keysetsMap.get(window).remove();
     }
     let sidebarKey;
-    commands.forEach((command, name) => {
+    for (let [name, command] of commands) {
       if (command.shortcut) {
         let parts = command.shortcut.split("+");
 
@@ -419,7 +415,7 @@ export class ExtensionShortcuts {
           sidebarKey = keyElement;
         }
       }
-    });
+    }
     doc.documentElement.appendChild(keyset);
     if (sidebarKey) {
       window.SidebarUI.updateShortcut({ keyId: sidebarKey.id });
diff --git a/toolkit/components/extensions/ExtensionStorageIDB.sys.mjs b/toolkit/components/extensions/ExtensionStorageIDB.sys.mjs
index 72a14ef9b223eb..3f338aae9076f4 100644
--- a/toolkit/components/extensions/ExtensionStorageIDB.sys.mjs
+++ b/toolkit/components/extensions/ExtensionStorageIDB.sys.mjs
@@ -2,7 +2,6 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-export let ExtensionStorageIDB;
 import { XPCOMUtils } from "resource://gre/modules/XPCOMUtils.sys.mjs";
 import { IndexedDB } from "resource://gre/modules/IndexedDB.sys.mjs";
 
@@ -238,10 +237,7 @@ class ExtensionStorageLocalIDB extends IndexedDB {
     // Explicitly create a transaction, so that we can explicitly abort it
     // as soon as one of the put requests fails.
     const transaction = this.transaction(IDB_DATA_STORENAME, "readwrite");
-    const objectStore = transaction.objectStore(
-      IDB_DATA_STORENAME,
-      "readwrite"
-    );
+    const objectStore = transaction.objectStore(IDB_DATA_STORENAME);
     const transactionCompleted = transaction.promiseComplete();
 
     if (!serialize) {
@@ -585,7 +581,7 @@ async function migrateJSONFileData(extension, storagePrincipal) {
  * This ExtensionStorage class implements a backend for the storage.local API which
  * uses IndexedDB to store the data.
  */
-ExtensionStorageIDB = {
+export var ExtensionStorageIDB = {
   BACKEND_ENABLED_PREF,
   IDB_MIGRATED_PREF_BRANCH,
   IDB_MIGRATE_RESULT_HISTOGRAM,
diff --git a/toolkit/components/extensions/ExtensionStorageSyncKinto.sys.mjs b/toolkit/components/extensions/ExtensionStorageSyncKinto.sys.mjs
index 95200a835e3e33..d10b140f7ef17e 100644
--- a/toolkit/components/extensions/ExtensionStorageSyncKinto.sys.mjs
+++ b/toolkit/components/extensions/ExtensionStorageSyncKinto.sys.mjs
@@ -373,6 +373,7 @@ async function storageSyncInit() {
   }
   return storageSyncInit.promise;
 }
+storageSyncInit.promise = undefined;
 
 // Kinto record IDs have two conditions:
 //
diff --git a/toolkit/components/extensions/ExtensionTelemetry.sys.mjs b/toolkit/components/extensions/ExtensionTelemetry.sys.mjs
index 9cdc2ec5111b52..06137b9a23fa2f 100644
--- a/toolkit/components/extensions/ExtensionTelemetry.sys.mjs
+++ b/toolkit/components/extensions/ExtensionTelemetry.sys.mjs
@@ -330,7 +330,7 @@ export var ExtensionTelemetry = new Proxy(metricsCache, {
     // telemetry histogram counterpart, we would need to change this check
     // accordingly.
     if (!(prop in HISTOGRAMS_IDS)) {
-      throw new Error(`Unknown metric ${prop}`);
+      throw new Error(`Unknown metric ${String(prop)}`);
     }
 
     // Lazily create and cache the metric result object.
diff --git a/toolkit/components/extensions/ExtensionTestCommon.sys.mjs b/toolkit/components/extensions/ExtensionTestCommon.sys.mjs
index 760eba7749f59e..94cb801cc5edf5 100644
--- a/toolkit/components/extensions/ExtensionTestCommon.sys.mjs
+++ b/toolkit/components/extensions/ExtensionTestCommon.sys.mjs
@@ -36,8 +36,6 @@ const { flushJarCache } = ExtensionUtils;
 
 const { instanceOf } = ExtensionCommon;
 
-export var ExtensionTestCommon;
-
 /**
  * A skeleton Extension-like object, used for testing, which installs an
  * add-on via the add-on manager when startup() is called, and
@@ -226,7 +224,7 @@ const ExtensionTestAssertions = {
 
     const { persistentListeners } = extension;
     if (
-      !persistentListeners?.size > 0 ||
+      !persistentListeners?.size ||
       !persistentListeners.get(apiNs)?.has(apiEvent)
     ) {
       return [];
@@ -283,7 +281,7 @@ const ExtensionTestAssertions = {
   },
 };
 
-ExtensionTestCommon = class ExtensionTestCommon {
+export var ExtensionTestCommon = class ExtensionTestCommon {
   static get testAssertions() {
     return ExtensionTestAssertions;
   }
diff --git a/toolkit/components/extensions/ExtensionWorkerChild.sys.mjs b/toolkit/components/extensions/ExtensionWorkerChild.sys.mjs
index 77089492f5d825..f93f6968e93cde 100644
--- a/toolkit/components/extensions/ExtensionWorkerChild.sys.mjs
+++ b/toolkit/components/extensions/ExtensionWorkerChild.sys.mjs
@@ -21,7 +21,7 @@ import {
 } from "resource://gre/modules/ExtensionPageChild.sys.mjs";
 import { ExtensionUtils } from "resource://gre/modules/ExtensionUtils.sys.mjs";
 
-const { BaseContext, defineLazyGetter } = ExtensionCommon;
+const { BaseContext, redefineGetter } = ExtensionCommon;
 
 const {
   ChildAPIManager,
@@ -154,14 +154,12 @@ class WorkerPort extends Port {
     api.portId = this.portId;
     return api;
   }
-}
 
-defineLazyGetter(WorkerPort.prototype, "api", function () {
-  // No need to clone the API object for the worker, because it runs
-  // on a different JSRuntime and doesn't have direct access to this
-  // object.
-  return this.getAPI();
-});
+  get api() {
+    // No need to clone this for the worker, it's on a separate JSRuntime.
+    return redefineGetter(this, "api", this.getAPI());
+  }
+}
 
 /**
  * A Messenger subclass specialized for the background service worker.
@@ -697,20 +695,19 @@ class WorkerContextChild extends BaseContext {
 
     super.unload();
   }
-}
 
-defineLazyGetter(WorkerContextChild.prototype, "messenger", function () {
-  return new WorkerMessenger(this);
-});
-
-defineLazyGetter(
-  WorkerContextChild.prototype,
-  "childManager",
-  getContextChildManagerGetter(
-    { envType: "addon_parent" },
-    WebIDLChildAPIManager
-  )
-);
+  get childManager() {
+    const childManager = getContextChildManagerGetter(
+      { envType: "addon_parent" },
+      WebIDLChildAPIManager
+    ).call(this);
+    return redefineGetter(this, "childManager", childManager);
+  }
+
+  get messenger() {
+    return redefineGetter(this, "messenger", new WorkerMessenger(this));
+  }
+}
 
 export var ExtensionWorkerChild = {
   /** @type {Map<number, WorkerContextChild>} */
diff --git a/toolkit/components/extensions/ExtensionXPCShellUtils.sys.mjs b/toolkit/components/extensions/ExtensionXPCShellUtils.sys.mjs
index 1ef9dcb56902a8..08df6b3c24ffd8 100644
--- a/toolkit/components/extensions/ExtensionXPCShellUtils.sys.mjs
+++ b/toolkit/components/extensions/ExtensionXPCShellUtils.sys.mjs
@@ -32,6 +32,13 @@ let BASE_MANIFEST = Object.freeze({
 });
 
 class ExtensionWrapper {
+  /** @type {AddonWrapper} */
+  addon;
+  /** @type {Promise<AddonWrapper>} */
+  addonPromise;
+  /** @type {nsIFile[]} */
+  cleanupFiles;
+
   constructor(testScope, extension = null) {
     this.testScope = testScope;
 
@@ -447,20 +454,13 @@ class AOMExtensionWrapper extends ExtensionWrapper {
     return super.unload();
   }
 
-  async upgrade(data) {
-    this.startupPromise = new Promise(resolve => {
-      this.resolveStartup = resolve;
-    });
-    this.state = "restarting";
-
-    await this._flushCache();
-
-    let xpiFile = lazy.ExtensionTestCommon.generateXPI(data);
-
-    this.cleanupFiles.push(xpiFile);
-
-    return this._install(xpiFile);
-  }
+  /**
+   * Override for subclasses which don't set an ID in the constructor.
+   *
+   * @param {nsIURI} uri
+   * @param {string} id
+   */
+  maybeSetID(uri, id) {}
 }
 
 class InstallableWrapper extends AOMExtensionWrapper {
@@ -574,6 +574,21 @@ class InstallableWrapper extends AOMExtensionWrapper {
 
     return this._install(this.file);
   }
+
+  async upgrade(data) {
+    this.startupPromise = new Promise(resolve => {
+      this.resolveStartup = resolve;
+    });
+    this.state = "restarting";
+
+    await this._flushCache();
+
+    let xpiFile = lazy.ExtensionTestCommon.generateXPI(data);
+
+    this.cleanupFiles.push(xpiFile);
+
+    return this._install(xpiFile);
+  }
 }
 
 class ExternallyInstalledWrapper extends AOMExtensionWrapper {
@@ -587,8 +602,6 @@ class ExternallyInstalledWrapper extends AOMExtensionWrapper {
 
     this.state = "restarting";
   }
-
-  maybeSetID(uri, id) {}
 }
 
 export var ExtensionTestUtils = {
@@ -769,7 +782,7 @@ export var ExtensionTestUtils = {
    *
    * @returns {XPCShellContentUtils.ContentPage}
    */
-  loadContentPage(url, options, ...args) {
-    return XPCShellContentUtils.loadContentPage(url, options, ...args);
+  loadContentPage(url, options) {
+    return XPCShellContentUtils.loadContentPage(url, options);
   },
 };
diff --git a/toolkit/components/extensions/NativeMessaging.sys.mjs b/toolkit/components/extensions/NativeMessaging.sys.mjs
index a0d75360054357..dcd8fe78078925 100644
--- a/toolkit/components/extensions/NativeMessaging.sys.mjs
+++ b/toolkit/components/extensions/NativeMessaging.sys.mjs
@@ -40,7 +40,15 @@ const PREF_MAX_READ = "webextensions.native-messaging.max-input-message-bytes";
 const PREF_MAX_WRITE =
   "webextensions.native-messaging.max-output-message-bytes";
 
-export var NativeApp = class extends EventEmitter {
+XPCOMUtils.defineLazyPreferenceGetter(lazy, "maxRead", PREF_MAX_READ, MAX_READ);
+XPCOMUtils.defineLazyPreferenceGetter(
+  lazy,
+  "maxWrite",
+  PREF_MAX_WRITE,
+  MAX_WRITE
+);
+
+export class NativeApp extends EventEmitter {
   /**
    * @param {BaseContext} context The context that initiated the native app.
    * @param {string} application The identifier of the native app.
@@ -153,7 +161,7 @@ export var NativeApp = class extends EventEmitter {
   static encodeMessage(context, message) {
     message = context.jsonStringify(message);
     let buffer = new TextEncoder().encode(message).buffer;
-    if (buffer.byteLength > NativeApp.maxWrite) {
+    if (buffer.byteLength > lazy.maxWrite) {
       throw new context.Error("Write too big");
     }
     return buffer;
@@ -174,9 +182,9 @@ export var NativeApp = class extends EventEmitter {
     this.readPromise = this.proc.stdout
       .readUint32()
       .then(len => {
-        if (len > NativeApp.maxRead) {
+        if (len > lazy.maxRead) {
           throw new ExtensionError(
-            `Native application tried to send a message of ${len} bytes, which exceeds the limit of ${NativeApp.maxRead} bytes.`
+            `Native application tried to send a message of ${len} bytes, which exceeds the limit of ${lazy.maxRead} bytes.`
           );
         }
         return this.proc.stdout.readJSON(len);
@@ -265,7 +273,7 @@ export var NativeApp = class extends EventEmitter {
 
     let buffer = msg;
 
-    if (buffer.byteLength > NativeApp.maxWrite) {
+    if (buffer.byteLength > lazy.maxWrite) {
       throw new ExtensionError("Write too big");
     }
 
@@ -380,17 +388,4 @@ export var NativeApp = class extends EventEmitter {
 
     return result;
   }
-};
-
-XPCOMUtils.defineLazyPreferenceGetter(
-  NativeApp,
-  "maxRead",
-  PREF_MAX_READ,
-  MAX_READ
-);
-XPCOMUtils.defineLazyPreferenceGetter(
-  NativeApp,
-  "maxWrite",
-  PREF_MAX_WRITE,
-  MAX_WRITE
-);
+}
diff --git a/toolkit/components/extensions/Schemas.sys.mjs b/toolkit/components/extensions/Schemas.sys.mjs
index 8eb16a6d20dcd7..ff7f926705e5d9 100644
--- a/toolkit/components/extensions/Schemas.sys.mjs
+++ b/toolkit/components/extensions/Schemas.sys.mjs
@@ -39,8 +39,6 @@ XPCOMUtils.defineLazyPreferenceGetter(
   false
 );
 
-export let Schemas;
-
 const KEY_CONTENT_SCHEMAS = "extensions-framework/schemas/content";
 const KEY_PRIVILEGED_SCHEMAS = "extensions-framework/schemas/privileged";
 
@@ -378,9 +376,12 @@ class Context {
       localize(value, context) {
         return value;
       },
+      ...params.preprocessors,
     };
+
     this.postprocessors = POSTPROCESSORS;
-    this.isChromeCompat = false;
+    this.isChromeCompat = params.isChromeCompat ?? false;
+    this.manifestVersion = params.manifestVersion;
 
     this.currentChoices = new Set();
     this.choicePathIndex = 0;
@@ -390,17 +391,6 @@ class Context {
         this[method] = params[method].bind(params);
       }
     }
-
-    let props = ["isChromeCompat", "manifestVersion", "preprocessors"];
-    for (let prop of props) {
-      if (prop in params) {
-        if (prop in this && typeof this[prop] == "object") {
-          Object.assign(this[prop], params[prop]);
-        } else {
-          this[prop] = params[prop];
-        }
-      }
-    }
   }
 
   get choicePath() {
@@ -1211,7 +1201,7 @@ const FORMATS = {
     // Our pattern just checks the format, we could still have invalid
     // values (e.g., month=99 or month=02 and day=31).  Let the Date
     // constructor do the dirty work of validating.
-    if (isNaN(new Date(string))) {
+    if (isNaN(Date.parse(string))) {
       throw new Error(`Invalid date string ${string}`);
     }
     return string;
@@ -2554,6 +2544,8 @@ class ValueProperty extends Entry {
 // Represents a "property" defined in a schema namespace that is not a
 // constant.
 class TypeProperty extends Entry {
+  unsupported = false;
+
   constructor(schema, path, name, type, writable, permissions) {
     super(schema);
     this.path = path;
@@ -2692,6 +2684,8 @@ class SubModuleProperty extends Entry {
 // care of validating parameter lists (i.e., handling of optional
 // parameters and parameter type checking).
 class CallEntry extends Entry {
+  hasAsyncCallback = false;
+
   constructor(schema, path, name, parameters, allowAmbiguousOptionalArguments) {
     super(schema);
     this.path = path;
@@ -3118,9 +3112,11 @@ class Namespace extends Map {
       this._lazySchemas.unshift(...this.superNamespace._lazySchemas);
     }
 
-    for (let type of Object.keys(LOADERS)) {
-      this[type] = new DefaultMap(() => []);
-    }
+    // Keep in sync with LOADERS above.
+    this.types = new DefaultMap(() => []);
+    this.properties = new DefaultMap(() => []);
+    this.functions = new DefaultMap(() => []);
+    this.events = new DefaultMap(() => []);
 
     for (let schema of this._lazySchemas) {
       for (let type of schema.types || []) {
@@ -3652,7 +3648,7 @@ export class SchemaRoot extends Namespace {
   }
 }
 
-Schemas = {
+export var Schemas = {
   initialized: false,
 
   REVOKE: Symbol("@@revoke"),