I am running mostly Firefox or Librewolf on Linux these days, but I do not remember having to enable it. Not all of my systems support accelerating AV1 in their hardware, but they do play at 1080p (but with framedrops once above 30fps on the unaccelerated computer).
But yeah, I do hope YT keeps VP9 around because of the acceleration.
I mean, given that many devices do not support accelerating it, it is in practice "hard to accelerate" unless you add a new gfx card or buy a new device.
I may not have worded it optimally (2L speaker), but I am sure it was fairly clear what I meant. 🙂
I can only imagine that they (OP) set quality settings on [auto]. That way they might have YT constantly lowering bitrates/resolution.
I do not have any issues either, but I use fixed quality settings.
Youtube pushes the AV1 "format" heavily these days which is hard to decode using hardware acceleration, given that a lot of devices still out there do not support that.