[tssge]

I've had a similar issue with YouTube and it turned out to be AV1. You seem to have quite new GPU compared to me so I guess AV1 isn't a likely culprit, but perhaps Firefox is unable to offload AV1 to the GPU even if supported?

For me this caused YouTube to use CPU for AV1 decoding (as my GPU has no decoding support), resulting in stuttering, dropped frames etc. I disabled AV1 support in my browser so that YouTube is "forced" to serve me VP9 and after that video decoding was offloaded to my GPU resolving the issue.

YouTube started serving AV1 by default rather recently for the higher bitrate quality settings.