Firefox doesn't play MP4 files in the "video" tag and doesn't fal back to the Flash Flowplayer in the object tag that is provided as an alternative.
You can try to right click the player and select View Video to see if that works.

Yes right clicking will play the file what does this mean?

Firefox can only play a few media file types via the video tag and its internal player.
For unsupported file types you can use View video or copy the video URL to the clipboard and play the video in an external player.

See:

• https://developer.mozilla.org/En/Media_formats_supported_by_the_audio_and_video_elements