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.
I use Librewolf with Ublock and sponsorblock. Sometimes youtube video stops loading or keeps seeking 5-10 seconds forward. However, reloading the page helps. I don’t have such problems when watching Youtube with mpv.