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.
When it all goes down, there’s always the option of just downloading what you want to watch and watch it in your favorite media player. That’s what I do on my phone since I refuse to watch ads and they make everything buffer. Hit the dl button and in under a minute i have the video downloaded in hd.