title: URLRequest failing on a mac on firefox
body: <p>I'm using a class called MultipartURLLoader which just uses URLRequest to POST a
  file to a url using the byte data.  It all works fine except in Firefox on a
  Mac.  It's fine in other browsers on a Mac and fine on a PC.  I have been
  debugging it and can see that the call to MultipartURLLoader works fine if you
  don't actually add the file, as soon as I include the "addFile" line it fails.
  The code is simply:</p>
  
  <pre><code>var ml:MultipartURLLoader = new MultipartURLLoader();
  ml.addEventListener(Event.COMPLETE, mp3Saved);
  ml.addEventListener(IOErrorEvent.IO_ERROR, ioErrorHandler);
  ml.addEventListener(SecurityErrorEvent.SECURITY_ERROR, securityErrorHandler);
  ml.addEventListener(Event.OPEN, openHandler);
  ml.addEventListener(ProgressEvent.PROGRESS, progressHandler);
  ml.addEventListener(HTTPStatusEvent.HTTP_STATUS, httpStatusHandler);
  ml.addFile(sba, new Date().time+'.mp3', 'audio_file');
  ml.load(url);
  </code></pre>
  
  <p>The other interesting thing is that none of those events get fired - it doesn't
  appear to open, report any progress, report any errors or receive any http
  responses.  However after a few minutes the error event is triggered so the
  flash is definitely attempting it and timing out.  I downloaded an add-on in
  Firefox called Live HTTP Headers so I could watch what was being posted.  And
  nothing is at all - the post is just not sent when I try to include the
  "addFile" line. </p>