I just installed and run my browser through a Fedora toolbox (distrobox) and it fixed that problem completely.

I use FreeTube for yt, but that has issues with IP blocks and stuff while using a VPN, but it works if I switch servers once or twice.

have been using bazzite for a while now and don't have issues with Firefox, could be tests Google are running. they recently are targeting users of adblockers and artificially setting high load times in hopes users disable them. a decent alternative is freetube (on flathub) which is anonymous so you can't login but you can import playlists and includes ad+sponsor block. they're pretty fast at fixing Google's attempts to break it too, sometimes you might have to change instances to get it working.

I yeeted the installed one and just went with the rpm Firefox, works a treat

I personally wouldn't describe it as horrible, but it is good to hear someone else seeing problems with video playback on YT. Just curious though, did you have any add-ons? I do and I need to compare playback without them. Hrmm

I recommend some variant of the h264ify plugin if you're having weird video issues with firefox based browsers.

There's some nonsense going on with some of the codecs Youtube uses + FF... YT used to let you more manually set your codec preferences, now they don't, but the h264ify plugins basically allow you to set those prefs yourself, and by default just disallow the problematic ones.