title: Navigator credentials creation method returning an exception
body: <p>Trying to run a demo for the webautn spec (<a href="https://www.w3.org/TR/webauthn/" rel="noreferrer">https://www.w3.org/TR/webauthn/</a>) available (<a href="https://github.com/molekilla/webauthn-demo-fork" rel="noreferrer">https://github.com/molekilla/webauthn-demo-fork</a>) under Firefox Nightly.</p>
  
  <pre><code>getMakeCredentialsChallenge({
          username,
          name
      })
      .then((response) =&gt; {
          console.log(response);
  
          let publicKey = preformatMakeCredReq(response);
  
          console.log(publicKey);
  
          console.log(publicKey.challenge)
  
          return navigator.credentials.create({publicKey})
      })
  </code></pre>
  
  <p>Whenever the execution reaches the return statement, the Promise will stay pending for a few seconds and eventually rejects, logging either <code>[Exception... "Abort"  nsresult: "0x80004004 (NS_ERROR_ABORT)"  location: "&lt;unknown&gt;"  data: no]</code> or <code>UnknownError: The operation failed for an unknown transient reason</code>. Both objects seem fine. Any idea about the reason why is it not resolving?</p>