Appending a dummy domain at end is a kind of a patch up and should be avoided.
To enable username login just follow these simple steps.
Sign Up
During sign up take the userid , email and password . Register the user with normal email and password. On Success of it save the email against the user_id in a separate node(branch).
mButtonSignUp.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
if(isValid()){
mProgressBar.setVisibility(View.VISIBLE);
final String userId = mEditTextUserId.getText().toString();
final String emailId = mEditTextEmail.getText().toString() ;
String password = mEditTextPassword.getText().toString() ;
firebaseRef.createUser(emailId, password, new Firebase.ResultHandler() {
@Override
public void onSuccess() {
firebaseRef.child("user_ids").child(userId).setValue(emailId);
Toast.makeText(getBaseContext(),"You are successfully registered ",Toast.LENGTH_SHORT).show();
mProgressBar.setVisibility(View.GONE);
}
@Override
public void onError(FirebaseError firebaseError) {
mProgressBar.setVisibility(View.GONE);
Toast.makeText(getBaseContext(),firebaseError.toString(),Toast.LENGTH_SHORT).show();
}
});
}
}
});
Database
Database structure will look like this

Login
Check if the user has entered an email or userId. If it is a email id then directly perform login with it otherwise fetch the email id associated with the username and perform login.
Button buttonLogIn = (Button)findViewById(R.id.button_login);
buttonLogIn.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
mProgressBar.setVisibility(View.VISIBLE);
String username = mEditTextEmail.getText().toString() ;
final String password = mEditTextPassWord.getText().toString() ;
// Check if it is an email or not
if(android.util.Patterns.EMAIL_ADDRESS.matcher(username).matches()) {
performLogin(username,password);
}else{
//get the emailId associated with the username
firebaseRef.child("user_ids").child(username)
.addListenerForSingleValueEvent(new ValueEventListener() {
@Override
public void onDataChange(DataSnapshot dataSnapshot) {
if(dataSnapshot!=null){
String userEmail = dataSnapshot.getValue(String.class);
performLogin(userEmail,password);
}
}
@Override
public void onCancelled(FirebaseError firebaseError) {
//Handle Error
}
});
}
}
});
private void performLogin(String emailId, String password) {
firebaseRef.authWithPassword(emailId,password, new Firebase.AuthResultHandler() {
@Override
public void onAuthenticated(AuthData authData) {
uid = authData.getUid() ;
Toast.makeText(getBaseContext(), authData.toString(), Toast.LENGTH_SHORT).show();
}
@Override
public void onAuthenticationError(FirebaseError firebaseError) {
Toast.makeText(getBaseContext(), firebaseError.toString(), Toast.LENGTH_SHORT).show();
}
});
}