handle situation where storage.sync is disabled in certain versions o…

…f firefox

app/scripts/background.js

@@ -47,11 +47,10 @@ async function initialize () {
 async function loadStateFromPersistence () {
   // migrations
   const migrator = new Migrator({ migrations })
-  // fetch from extension store
-  const extensionData = await extensionStore.fetch() // TODO: handle possible exceptions (https://developer.chrome.com/apps/runtime#property-lastError)
   // read from disk
   let versionedData = diskStore.getState() || migrator.generateInitialState(firstTimeState)
-  // merge extension and versioned data
+  // fetch from extension store and merge in data
+  const extensionData = await extensionStore.fetch() // TODO: handle possible exceptions (https://developer.chrome.com/apps/runtime#property-lastError)
   versionedData = { ...versionedData, ...extensionData }
   // migrate data
   versionedData = await migrator.migrateData(versionedData)

app/scripts/lib/extension-store.js

@@ -1,11 +1,24 @@
 const extension = require('extensionizer')
 
 const KEYS_TO_SYNC = ['KeyringController', 'PreferencesController']
+const FIREFOX_SYNC_DISABLED_MESSAGE = 'Please set webextensions.storage.sync.enabled to true in about:config'
+
+const handleDisabledSyncAndResolve = (resolve, toResolve) => {
+  // Firefox 52 has sync available on extension.storage, but it is disabled by default
+  const lastError = extension.runtime.lastError
+  if (lastError && lastError.message.includes(FIREFOX_SYNC_DISABLED_MESSAGE)) {
+    resolve({})
+  } else {
+    resolve(toResolve)
+  }
+}
 
 module.exports = class ExtensionStore {
   async fetch() {
     return new Promise((resolve) => {
-      extension.storage.sync.get(KEYS_TO_SYNC, data => resolve(data))
+      extension.storage.sync.get(KEYS_TO_SYNC, (data) => {
+        handleDisabledSyncAndResolve(resolve, data)
+      })
     })
   }
   async sync(state) {
@@ -14,7 +27,9 @@ module.exports = class ExtensionStore {
       return result
     }, {})
     return new Promise((resolve) => {
-      extension.storage.sync.set(dataToSync, () => resolve())
+      extension.storage.sync.set(dataToSync, () => {
+        handleDisabledSyncAndResolve(resolve)
+      })
     })
   }
 }