I'm running Firefox 84.0 on Ubuntu 20.04. Everything was working fine until a couple of weeks ago when Firefox just stopped being able to play videos. It doesn't matter whether the site is YouTube or Crunchyroll, or what the format is.
If I visit a page with a video, for example a YouTube site, it spins and eventually tells me, "If this video doesn't play after a few seconds, try restarting your device." In the console I see errors like Cannot play media. No decoders for requested formats: audio/webm; codecs="opus", video/mp4; codecs="av01.0.04M.08"
(or some other codec - it doesn't appear to matter what it is).
I've tried refreshing Firefox, launching under a new profile, clearing the cache (and cookies), installing/uninstalling/reinstalling ffmpeg and libavcodec, turning off "Play DRM protected content" between restarting firefox, launching in safe mode, and of course rebooting. Nothing seems to work.
How do I get video playback to work again in Firefox? Is there some log somewhere that will tell me what's going on?