I had the same problem. Make sure you're running the latest version of uBlock Origin. By that I mean you should explicitly check for updates and install the latest version. That fixed it for me.

I've disabled adblock/vpn and can watch maybe 1-2 videos before the video player starts blacking out on each one. Only restarting firefox "resets" it and I haven't been able to find a reason for this happening. Time for yt-dlp I guess

I disable youtube instead when this happens.

If I care enough I'll yl-dlp it to my jellyfin server, but yeah I mostly just don't bother.