title: CORS withCredentials XHR preflight not posting Cookies in Firefox
body: <p>I'm trying to do a CORS XHR post w/ credentials.  It works great in Chrome, but not in Firefox.  The cookies are not present in the pre-flight request headers, and so I'm seeing a 302. This works perfectly in Chrome, as cookies are in the pre-flight request headers and the subsequent POST goes through. </p>
  
  <p>Why wouldn't this work in FF? What am I missing?</p>
  
  <pre><code>// assume url, boundEventHandler and uploadData are defined, as this definitely works in Chrome
  var xhr = new XMLHttpRequest(); 
  xhr.open("POST", url, true); 
  xhr.addEventListener ("readystatechange", boundEventHandler, false); 
  xhr.withCredentials = true;  // FWIW, I've also tried the string 'true'
  xhr.send(uploadData);
  </code></pre>
  
  <p>Any ideas?  I see some posts that say I can proxy the request on the server side, but I'd prefer to get this working in accordance w/ the CORS spec.</p>
  
  <p>Thanks!</p>