This is caused by the limited support for the MP4 format within the video tag in Firefox. Support was not added until Firefox 21, and it is still limited to Windows 7 and above. The main reason for the limited support revolves around the royalty fee attached to the mp4 format.
Check out Supported media formats and Media formats supported by the audio and video elements directly from the Mozilla crew or the following blog post for more information:
http://pauljacobson.org/2010/01/22/2010122firefox-and-its-limited-html-5-video-support-html/