if the website is trying to get the video as https, while your server is only serving http, that's not going to work.
But even if you fix this and load with http, you'll probably get the error that Leo said. That's a limit of the browser.
By the way, a little offtopic with Mozilla experience: in 2.3m and HFS 3 all unprotected .mp4 videos are opened as blank page with audio only and player controls (Opera and Chrome are fine). Do you have idea what to do?
mp4 is not always the same, you can have files requiring different "codecs".
Your browser will support only some, and another browser may support different ones.
So a file that's played in Chrome may not be played in Firefox, but also some files won't play in no browser.
If you get the audio only, it's because video and audio codecs are different, so Firefox is apparently supporting that specific audio codec but not the video codec.
There's no solution that I know. In time a browser can support more formats.