How to get current user id on firebase

I have an authentication flow with twitter in an Ember application.

I would like to have the unique user id used in Firebase. eg -KGRt7sTuWbC9qsqQmuP here

{
      "body" : "test",
      "date" : "2016-04-28T12:42:44.401Z",
      "title" : "lorem ipsum",
      "titleURL" : "lorem_ipsum",
      "user" : "-KGRt7sTuWbC9qsqQmuP"
}

with

var ref = new Firebase("<<firebase-app-name>>.firebaseio.com");
var authData = ref.getAuth();
if (authData) {
    console.log("Authenticated user with uid:", authData.uid);
}

i get the twitter id.

How to get the real id?