Most articles indicate that Chromium performs the best on the Pi but bear in mind that most people refer to Raspbian and the chromium package there has RPI-specific patches.
Yesterday I installed Arch Linux (base Arch not Manjaro) on an RPI4 and while I didn’t get the performance others claim with Chromium on Raspbian (or RaspberryPi OS, whatever) it was still better than Firefox (not just Youtube, the browsing experience in general).
I did gather a list of CLI arguments to launch it though that I’ve added on top of the ones that came by default, and I did mess with /boot/config.txt to try to tweak the GPU performance (including overclocking)
Oh and I’ve installed the “Iridium for YouTube” extension in order to disable high framerate playback (60 FPS) since I’ve experience a lot of interruptions with those.
Other than that, I haven’t been able to do much better so far. We’ll just have to wait for the software to improve. The hardware is more than capable of doing it, I’ve had even RPI3’s that played video better than this when running media center OSes.
And as @Strit said, 64bit is early and experimental for the Pi - which is easy to overlook but is actually very important to keep in mind