Why does Firebase auth work for Chrome but not Firefox?

Question

Firebase onAuthStateChanged() is returning user credentials in Chrome, but not in Firefox:

What is the likely reason(s) for this?

Update: Here is the minimum code needed to reproduce the issue:

var firebase = require("firebase/app"),
  C = require("./constants.js");
require("firebase/auth");

firebase.initializeApp(C.FIREBASE_CONFIG);

firebase.auth().onAuthStateChanged(function(user) { 
    console.log('user is ' + user);
})

I have a lot more code in my app but after stripping everything out, this is the only code running and the issue still occurs:

Answer 1

Perhaps you have enabled Enhanced Tracking Protection in your Firefox.

It needs to be disabled for the site or globally for Firebase Authentication to work. If Enhanced Tracking Protection is enabled, onAuthStateChanged is called with null, even when you authenticated successfully.

See Firefox documentation for configuration: https://support.mozilla.org/en-US/kb/enhanced-tracking-protection-firefox-desktop