So its super unstable, but I've installed firefox on the pi 4, and when it behaves, it can do windowed youtube playback without dropping frames, and can go fullscreen with some stuttering but according to the player, still no dropped frames. Best I've gotten to play is 720p, 1080p causes the player to crash.

Running firefox 60.7.2esr 32 bit since that's all I could work out how to install lol

Can't wait for updates to add actual optimization and hardware utilization