There seems to be some issues with your files themselves. This is the file that you have set: http://wac.c20c.edgecastcdn.net/00C20C/FTP_videos02-19-2014/ChicagoFire-KeepYourMouthShut.mp4
but it gives a 404 error and isn’t working on any browser.
What is the link to the page that is breaking on Firefox?
Thanks for providing a new link. It appears to be an issue with the codec of your MP4 file. I set the primary mode to flash and it breaks in Chrome and Safari as well. Therefore, this isn’t just a browser compatibility issue.
I would suggest re-transcoding the file to see if this fixes the issue. Here is a valid mp4 file that works in HTML5 and flash if you need one for testing purposes: http://qa.jwplayer.com/support-demos/static/bunny.mp4
Hope this helps! Let me know if you have any more questions.