Firebase: This domain is not authorized

Question

I imported Firebase JS v3 API and used to initialize Firebase using:

firebase.initializeApp(config);

However the app failed to load correct

Answer 1

None of the above answers worked for me, as I already had localhost in Authorized domains list. For me the problem was incorrect API key, I must have somehow deleted one of the characters from it. I got a more descriptive (or rather - not misleading) error message when I changed to signInWithPopup to signInWithRedirect. Possibly the same problem might happen when the API key is expired.