Some Firefox users noticed playback issues on YouTube for several months. These affected high resolution videos only, from 1080p and up. To make matters worse, no clear pattern could be identified.
Some videos played fine, others would stop abruptly when they ran out of buffer.
Personally, I copy the url from the link or at worst the url bar after clicking into youtube.
Then ‘mpv + [pasted url]’ into a terminal (I use Yakuake as a quick terminal) and it’s away quicker than youtube.com anyway.
mpv automagically invokes ytdl/yt-dlp if you have it installed on linux or if it’s in the same directory as mpv or in a directory added to your windows search paths on windows. MacOS, just use a package manager like macports/homebrew.