Go to Firebase Authentication configuration (/admin/config/system/firebase_authentication)
Configure your private key path. E.g: ../../private/your-firebase-private-key.json
Access the REST resources pages and enable the following REST endpoints (the module already came with pre-configured settings - Don't set the authentication provider on these endpoints to the firebase_authentication)
Firebase Authentication user register
Firebase Authentication user login
Configure the permissions to use the REST endpoints above.