Start Firefox in Safe Mode to check if one of the extensions (Firefox/Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem (switch to the DEFAULT theme: Firefox/Tools > Add-ons > Appearance).
Do NOT click the Reset button on the Safe mode start window or otherwise make changes.
Re: Firefox won't play a video on my website and it is not flash.
Thank you for the response. I did everything you suggested. And then I went to Firefox and did all the troubleshooting suggestions they have. And it still does not play. I tried it on my laptop and it works fine. It's only on my desktop and only in Firefox. Do you know what else could be the problem?