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 your experience to only be degraded a little.
For an slightly better but still degraded experience they want you on Google Chrome. But remember please use 360p or lower so their poor servers don’t have to work so hard, they even help you do this by default on mobile!