I'm using Firefox v 83.0 on Manjaro (Gnome) Linux. YouTube videos in theater/full-screen mode lag if resolution is higher than 360p and cause the CPU usage to burst. This happens for large web pages as well: when I scroll the page, text overlaps and everything is terribly slow. I've checked and this seems to be a Firefox related problem; videos play smoothly both on VLC and Chromium. I've tried all official troubleshooting suggestions: changing to default theme, refreshing Firefox, starting in safe-mode, disabling extensions, disabling hardware acceleration and so on, with no success. I've never had any problems with Firefox for Linux before and I wonder what could be the cause.

I attached screenshots for comparison of CPU usage for the same video playing in theater mode in Firefox and Chromium.