It’s probably the Flash plugin for Firefox… seen it before. Try these help docs:
https://helpx.adobe.com/flash-player/kb/video-playback-issues.html#main_Solve_video_playback_issues
https://support.mozilla.org/en-US/kb/fix-common-audio-and-video-issues
I would look at deleting Flash from your control panel… reinstalling using these steps. You had no public link for your site… so I can’t see the settings on the video embed elements.
Oh also… it may be a good idea to add a ‘page loader’ if you’re going to keep all those videos on the Media page. Or you could use collections… and add video frame divs/link blocks with an image hover … and users can click to view in a medium lightbox. I think that would help reduce the page load/size. Take care.