Polymer Firebase : Uncaught TypeError: Cannot read property 'signInWithPopup' of undefined

I'm a newbie with Polymer and Firebase, i am practicing with the two technologies. I am trying to implement firebase-auth for application authentication but i go a "Cannot read property 'signInWithPopup' of undefined" error.

i have this code :

<paper-toolbar>
  <span class="title">Sample!</span>
  <paper-icon-button
    icon="[[statusIcon(signedIn)]]"
    on-tap="processAuth">
  </paper-icon-button>
</paper-toolbar>
<firebase-auth
  id="auth"
  app-name="emotions"
  provider="google"
  signed-in=""
  user="">
</firebase-auth>

and the script for processAuth

processAuth: function() {
    this.$.auth.signInWithPopup();
}

and the following components were imported already:

<link rel="import" href="../../bower_components/polymerfire/polymerfire.html">
<link rel="import" href="../../bower_components/polymerfire/firebase-auth.html">

Did i missed something?

Sorry for this very noob question.