title: mp3 support on Firefox MediaSourceExtension
body: <p>I'm looking into implementing adaptive and progressive audio streaming in the browser, with no plugins. 
  MSE is the HTML5 API I was waiting for, available in FF 42, but it seems that the audio format support in Firefox is not there?... 
  mp3 audio is not working when using the MSE APIs.</p>
  
  <p>Here's a code snippet:</p>
  
  <pre><code>var mediaSource = new window.MediaSource();
  var audioSourceBuffer;
  
  mediaSource.addEventListener('sourceopen', function (e) {
      try {
          var mimeType = "audio/mpeg";
          audioSourceBuffer = mediaSource.addSourceBuffer(mimeType);
      } catch (e) {
          log('Exception calling addSourceBuffer', e);
          return;
      }
  }
  </code></pre>
  
  <p>I get a NotSupportedError exception when calling addSourceBuffer.</p>
  
  <p>Doesn't Firefox support mp3? from MDN list of supported formats (<a href="https://developer.mozilla.org/en-US/docs/Web/HTML/Supported_media_formats" rel="noreferrer">https://developer.mozilla.org/en-US/docs/Web/HTML/Supported_media_formats</a>) it implies that mp3 support should be there if the OS supports it - and the OS I am testing on (OSX) does support it.</p>
  
  <p>Any help appreciated!</p>