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.
For months I’ve had problems with YouTube videos in Firefox, at any resolution. They would play at really low FPS. Sometimes the player would pause and continue at the correct FPS, but usually not. Skipping backwards or forwards usually fixes it but not always. Another step in their monopoly to destroy competition.
Firefox for Android? Happens with any video, this is not YT bug.