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.
You can get an extension to easily change your browser ‘user agent’ to make YouTube think you’re on chrome lol
Not OP, I use this https://addons.mozilla.org/en-US/firefox/addon/uaswitcher/