Custom authDomain in Firebase

The authDomain relies on specific scripts being available on that domain. If your Single-Page App is hosted on Firebase Hosting with a custom domain, you will be able to use that domain as the authDomain.

Alternatively, you could set up a custom domain for Firebase Hosting on a subdomain of your domain e.g. auth.foobar.com and you'd then be able to use auth.foobar.com as your authDomain.

There is currently no support for using a non-Firebase-Hosting domain as your authDomain.

I saw a lot of answers in Google OAuth 2 authorization - Error: redirect_uri_mismatch that pointed to the need to set up custom domains authorization, checking HTTP/S URI schemes, etc (I had followed most of them, including making sure my custom domain was authorized under Firebase Hosting, Firebase Authentication and even on the GCP Identity Platform "Authorized Domains" settings (https://console.cloud.google.com/customer-identity/settings?project=\<your-gcp-project-id>). That is, none of the linked answers seemed to be specific to Firebase Auth so here is my experience, FWIW:

The following Authorisation Error message was showing up on the "Sign in with Google" popup:

Error 400: redirect_uri_mismatch
The redirect URI in the request, https://<project-id>.firebaseapp.com/__/auth/handler, does not match the ones authorized for the OAuth client. To update the authorized redirect URIs, visit: https://console.developers.google.com/apis/credentials/oauthclient/${your_client_id}?project=${your_project_number}

After a lot of digging, I realized I needed to set the value of "Authorized redirect URIs" to https://<my-auth-subdomain>.mydomain.org/__/auth/handler, referencing my custom domain. See the below attachments.

It wasn't immediately obvious to me, but I also had to make the above settings on the OAuth 2.0 Client ID that was auto generated by Google Service. Following this related answer, I had initially created a new client ID that didn't end up making any difference.

Go to Authentication --> Sign-in Method, you will put you custom domian under Authorized domains section