title: firebase auth with MIcrosoft Graph (accessToken)
body: <p>I am super hopeful someone can help me - I'm kind of stuck.</p>
  
  <p>I'm happily using firebase auth with Microsoft AD. My AuthProvider is <code>firebase.auth.OAuthProvider('microsoft.com')</code>.</p>
  
  <p>When I call <code>firebase.auth().signInWithPopup()</code> with that provider, everything works GREAT. I can pick out the <code>accessToken</code> from the resulting <code>UserCredential</code> and access Microsoft Graph api's no problem (yay!).</p>
  
  <p>Firebase persists and renews the authentication and my app gets the expected callback via <code>onAuthStateChanged</code> with the new <code>firebase.User</code> when the user returns to my SPA later (also yay!).</p>
  
  <p>The <em>bad news</em> (where I'm stuck) is: <em>how do I get the Microsoft Graph <code>accessToken</code> in this flow</em> (e.g. when the user returns to my app later)? I don't want them to have to re-authenticate with another popup (yech).</p>
  
  <p>Basically, how do I go from a valid <code>firebase.User</code> to a MS Graph <code>accessToken</code> when the user returns?</p>
  
  <p>Thanks so so much for any help!</p>