title: signInWithCustomToken() with Firebase's own tokens
body: <p>I want to authenticate users on multiple websites via sharing the JWT token. The token was generated originally by firebase, so I assume it is a good and valid token (tested on jwt.io and seems to be fine). I get it by <code>user.getToken()</code>.</p>
  
  <p>As I call <code>signInWithCustomToken(token)</code> I always get an error <code>auth/invalid-custom-token</code> with a message <code>"The custom token format is incorrect. Please check the documentation."</code>.</p>
  
  <p>What is strange, it does a HTTP POST request to <code>https://www.googleapis.com/identitytoolkit/v3/relyingparty/verifyCustomToken...</code> which returns HTTP 400. I have tried on both localhost and also on the https live website... but same result. I'm using Firebase 3.3.</p>
  
  <p>Any ideas, what could be wrong? I was using this feature with the old Firebase.</p>