I have this problem as well, and with the video upload website BitChute, starting with 66. I've been using FF since it's inception. Except for the recent add-on debacle (lt's kinda funny now), am mostly happy with the browser itself.
Twitter: Some videos play fine, others do not, it appears to be related to what the cell videographer used to film or upload the video with, but that's a guess. No GIF play at all. When scrolling down a thread the top edge of a GIF briefly flashes the true image, sometimes including the blue play button mid-image, usually not, and defaults to a black square with the GIF bug lower left corner.
BitChute: I login, allow all scripts to run, change FF default of autoplay audio Off to On, navigate to a notification of a new video upload, land on the page, hear the video but see nothing at all, not even a placeholder. The Disqus comments also do not display. There are no page elements displayed, either. Just blank space below the BitChute page header. I have filed a support ticket at BitChute but have not heard back now two weeks hence.
I have tried Ctrl+F5, no good. I tried unchecking Acceleration Hardware, no good. I checked about:config for Media settings, all appear to be at Default/Normal. I tried turning off add-ons one at a time, no good. I checked my anti-virus for any settings that might be offensive, none in Norton that I could find.
While I can live without the Twitter GIFs, I need to access the content on BitChute.
I will appreciate any help and am happy to assist in any way I can. First time bug reporter, so please forgive any posting goof up.