October 9, 2021 / October 16, 2021: Fixed some wording
The version of Firefox I'm using while writing this is: 78.15.0esr. I tested 720p and 1080p.
Links: YouTube full screen 1080p@60fps videos choppy (Mozilla Support)
Arch Wiki (1.3 Enable Legacy OpenGL compositor)
I think but cannot be sure that this might fix screen tearing as well. Here's a video to test it out with. COSTA RICA IN 4K 60fps HDR (ULTRA HD)