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.
It seems Netflix is able to detect that spoofing, and sends this error when trying to play videos, “Sorry, something went wrong. Please try again later.”. I had to disable my add-ons one by one to identify that it was User Agent Switcher causing it.