Install

Symfony

composer install
cp .env .env.local

Google Auth

Recommended way

Create a new Firebase project on the Firebase console
In the Authentication menu, choose the Sign-in method tab and enable the Google provider.
You will see the associated Web client id (GOOGLE_CLIENT_ID) and Web client secret (GOOGLE_CLIENT_SECRET), which you need to put into your .env.local file to the corresponding variables.
After this when you try to sign in with google, an error will arise about the redirect url. Follow the link to the Google cloud console and add the redirect to the Authorized redirect URIs list for your web client API key. The correct redirect url is in the error message too (http://yoururl/login-check)
On this admin (Google Cloud Console), you will have to create a Service account key for Firebase. In the Credentials menu, press the Create credentials dropdown button and choose Service account key. Choose a service account (if you created the project from firebase, then you will already have one) and create a JSON key. Put the json into the local var folder and name it exactly firebase-service-account.json. Or if you use Google App Engine, you can use the autodiscovery feature.

Separate Firebase and Google projects (advanced)

If you create a separate Firebase and separate Google Cloud project, then you will have to add your Google project's client ID to the Firebase Google Login client id whitelist on the Firebase console (Credentials > Sign-In Methods > Google)!

Facebook Auth

Create a new app in the Facebook Developers page. The app can stay in sandbox mode.
Enable facebook login, and copy the App ID (FACEBOOK_APP_ID) and App Secret (FACEBOOK_SECRET) into the .env.local file's corresponding variables
In the Facebook Login admin, add the redirect url: https://yoursite/facebook-callback. It must be https, otherwise it won't work sadly!
Enable Facebook login in the Firebase console: Authentication > Sign-in method > Facebook.

Where to look

The most interesting part is in the Security folder.

There is a common Symfony security user, which is the same for Google and Facebook login. It holds the LinkProviderData and the UserRecord data so they are serialized to the session and are available on every request.

The login happens in the Guard classes, where linkProviderThroughAccessToken and linkProviderThroughIdToken are called. Before that, the official SDK-s are used for log in.

After a successful login, the UserProvider checks (verifies) the firebase ID token on every request, which originated from the linkProviderThrough* calls.

On logout, we try to revoke all refresh tokens in the LogoutSuccessHandler, that's all what Firebase can do sadly.

If the login was successful, the logged in User's data is dumped and it is possible to log out.