diff --git a/src/cmd/run.js b/src/cmd/run.js
index 8c06f83702..3fc07dcab9 100644
--- a/src/cmd/run.js
+++ b/src/cmd/run.js
@@ -333,6 +333,6 @@ export class ExtensionRunner {
 
   run(profile: FirefoxProfile): Promise<FirefoxProcess> {
     const {firefoxApp, firefox} = this;
-    return firefoxApp.run(profile, {firefox});
+    return firefoxApp.run(profile, {firefoxBinary: firefox});
   }
 }
diff --git a/src/firefox/index.js b/src/firefox/index.js
index 4eed257226..c2330fe9e9 100644
--- a/src/firefox/index.js
+++ b/src/firefox/index.js
@@ -74,7 +74,7 @@ export function defaultRemotePortFinder(
 // Declare the needed 'fx-runner' module flow types.
 
 export type FirefoxRunnerParams = {
-  binary?: string,
+  binary: ?string,
   profile?: string,
   'new-instance'?: boolean,
   'no-remote'?: boolean,
@@ -105,7 +105,7 @@ export type FirefoxRunnerFn =
 export type FirefoxRunOptions = {
   fxRunner?: FirefoxRunnerFn,
   findRemotePort?: RemotePortFinderFn,
-  firefoxBinary?: string,
+  firefoxBinary: ?string,
   binaryArgs?: Array<string>,
 };
 
diff --git a/tests/test-cmd/test.run.js b/tests/test-cmd/test.run.js
index 7293530a57..c726aa8f3d 100644
--- a/tests/test-cmd/test.run.js
+++ b/tests/test-cmd/test.run.js
@@ -120,7 +120,7 @@ describe('run', () => {
 
     return cmd.run({firefox}).then(() => {
       assert.equal(firefoxApp.run.called, true);
-      assert.equal(firefoxApp.run.firstCall.args[1].firefox,
+      assert.equal(firefoxApp.run.firstCall.args[1].firefoxBinary,
                    firefox);
     });
   });