Firebase Username/Password Authentication - allow only one device to sign-in at a time [duplicate]

Answer 1

Revoke refresh token

FirebaseAuth.getInstance().revokeRefreshTokens(uid);

On revocation, the user is signed out and prompted to reauthenticate.

Answer 2

I don't think is it possible without a database, because by default firebase doesn't provide any way to limit or get active sessions for a user. What you can do is, after logging in you can maintain the login history of the user for different devices and also logout by revoking refresh tokens for that devices. Revoke refresh token

Hope this helps!