Does Firefox support Flowplayer?
There are 2 sites i have tried to use that use Flowplayer and I am unsuccessful in getting Firefox to play the MP4 files.
There seems to be no support info on this trouble.
Try to set the Boolean pref media.windows-media-foundation.enabled to false on the about:config page to disable built-in HTML5 media player.
You can open the about:config page via the location bar