That’s interesting. I wonder if this bugs affects both MP4 and WebM. (I would test this myself, but I’m currently using Abrowser, which is based off of Firefox. That said, the Abrowser developers tend to have a fall slightly behind in development: the latest version of Abrowser is based off of Firefox 43.)
If it only affects MP4, I’m guessing there’s some kind of issue with the H264 decoder built into Windows, in which case, one could simply disable H264 support in about:config. I’m not quite sure how to do this in Windows, since I use GNU/Linux, which has different H264 settings. However, if you search for Media Foundation and DirectShow, you’ll probably find the boolean options you want. You can then set these option to false, and re-test. Of course, you’ll probably want to change these values back to their default when the next version of Firefox come out, since the issue will likely have been fixed.
Please note that disabling H264 will likely break HTML5 playback on any YouTube video that doesn’t support WebM (yes, there are a few such videos), although Firefox should still be able to fall back on Flash Player. Also, disabling H264 will break HTML5 videos on other sites, however Firefox will probably be able to use Flash Player on those sites, as well.