@user2004685 answer above gives very good hint. But it does not work at least for the latest firebase-ui because currentUser.getProviderData().get(0).getProviderId() returns 'firebase'.

So the updated solution is

protected void onActivityResult(int requestCode, int resultCode, Intent data) {
  if (requestCode == RC_SIGN_IN) {
    IdpResponse response = IdpResponse.fromResultIntent(data);

    /* Success */
    if (resultCode