I'm hosting mp4 videos on my university server and when I try to play them in Firefox, I get the following error "No video with supported format and MIME type found". I can play those videos on Chrome and Opera, so I though it may be Firefox issue, but the same videos hosted on my private VPS server works fine also on Firefox. What may be the reason?
You should check if your server properly configured and sending right MIME type for video files you serve. For MP4 its generally video/mp4. As for "its works in Chrome" - Firefox is picky with mime types.