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.
My guess is that it’s supposed to look good if you have HDR enabled (which gives you 10 bits per color channel, although even with HDR, the desktop UI still renders in 8 bits per channel).
But still, it looks terrible for most people, and is a complete waste of resources. Because of the horrible banding, I initially thought something was wrong with my monitor. I don’t understand why they added this feature.