Problems with Firefox 3.6.13

Using KalturaCE hosted edition on site, I've found that Firefox can't play videos from it.

It shows "Sorry, clip not found", while other browsers are playing video seamlessly.
Inspecting this problem had shown me, that actually Firefox is loading video file, while (possible) can't play it using Kaltura player. And format of this video is mp4.

You can see this problem here:
http://www.buzzinefilm.com/interviews/film-buzzscene-tron-legacy-red-car...

Thanks!