Running on EndeavourOS (Arch base), Ryzen 5800X, AMD RX 6700XT, 32GB RAM, and a fiber connection that's getting 340Mbps down/90 Mbps up at my computer using fast.com speed test. Youtube and Twitch both completely bog down when setting video quality to 1080p60 on this system, but don't bog down on my both-feet-in-the-grave old Surface 3, nor does any other computer in the house bog down (all running Windows 10).

I've tested it both in xwayland and full wayland protocols, both have the exact same issue. I've not tested any other Linux distro. In about:preferences, I have tested all possible combinations of performance settings (Use recommended, use hardware when available, use neither), and no discernible difference between them, still no video at 1080p possible without insane buffering.

Edit here to add - this atrociously slow performance is only on YT and Twitch. Everything else absolutely flies, including gaming latency.

Second edit here - Just tested Chromium on the same system with 1080p video from YT and Twitch, it was buttery smooth with no buffering or stuttering. This leads me to believe it's Firefox specific somehow. My extensions are uBlock Origin, no$cript, BetterTTV, 7tv, and Dark Reader. Tested all extensions on/off in combo, results the same.