I have similar issues and it seems to be libnss3 3.72, but not entirely sure yet. My issue is that video playback will completly stop working atleast for youtube and twitch, haven't tested other sites yet. This started about a week ago. No error message to be found anywhere.
It seems to be triggered by doing stuff in other tabs. I can watch youtube/twitch for hours if not doing anything else, but then i do stuff in other tabs and video playback suddenly breaks. This can happen even while a video is running and it will just stop midway and then no new video can be opened and indeed in youtube's case it will just show a loading circle. This is happening atleast a few times a day and firefox needs to be restarted. It's not just video playback either. I noticed it happens a lot while being on aliexpress. I had it happen multiple times on the login page or the refund page and pulldown menus/select boxes also stop working at the same time video playback stops. I had to restart firefox 10+ times just to get 2 refunds done, with or without video's playing. Downgrading to firefox to 93 didn't help but also downgrading libnss3 to 3.70 seems to have fixed it (can't test firefox 94 with libnss3 3.70 as it depends on 3.72). Not sure yet tho, maybe someone else can confirm so this bug can be switched/tagged to libnss3.
