I use Firefox Nightly (which despite the name usually rolls out two versions a day!) and they fixed that a while ago. I don't know when the change will be in the standard release but you could download Nightly if you want that fix. The jittery controls are still present though.Working fine but no sound. If you try to play youtube video, it is muted
However ... I'm not sure if you can go back to the standard release once your profile has been used by Nightly. You should back it up first.
1647816 - MP4 fail to play in macOS Big Sur 11.0 Beta (20A4299v)
VERIFIED (haftandilian) in Core - Audio/Video: Playback. Last updated 2021-01-04.
bugzilla.mozilla.org