No longer works. YouTube probably changed something yet again, and broke it. This is the problem with unmaintained add-ons, they quickly become obsolete. Sadly there are no functioning alternatives, so I'm forced by YouTube to use the AV1 codec, even though my hardware is too slow to use it without stuttering. Even setting media.av1.enabled to false has no effect any more. Mozilla seems to think that every PC user can afford 2020 hardware. At this rate, I'm just going to have to stop using YouTube, unless Google/Alphabet/Whatever wants to send me a new PC.