User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_9_5) AppleWebKit/600.1.17 (KHTML, like Gecko) Version/7.1 Safari/537.85.10 Steps to reproduce: I created a simple <video> based web page that refers to an H.264-in-MP4 file. I also enabled the media.peerconnection.video.h264_enabled setting in about:config and restarted the browser. Then I opened the attached HTML file. (The test results are the same whether you open the HTML file locally or via a web server.) Actual results: On a Windows box with Firefox 33, the video plays just fine. (It also succeeds in Chrome on Windows, Mac OS X, Android and iOS; in Safari on OS X and iOS; and in recent versions of IE on Windows.) Of the desktop platforms I've tested, only Firefox 33 on Mac OS X fails. I get a dark gray error box in the browser window where the video should go, with the complaint "Video format or MIME type is not supported". The developer error console says: HTTP "Content-Type" of "video/mp4" is not supported. Load of media resource http://my-server/video/simple.mp4 failed. Expected results: Firefox 33 on Mac OS X should behave the same as Firefox 33 on Windows. That is, it should play this file. I cannot provide the actual MP4 file for copyright reasons, but I have tested this with dozens of MP4 files, and all show this symptom. I therefore assume you can use any H.264-in-MP4 file you have handy as "simple.mp4" for this test.

according to bug 1043696, h264 playback is not ready in Firefox33 OSX. media.peerconnection.video.h264 is only for webrtc and not for the <video> tag