[kiwijamo]

That sounds very strange. Certainly don't see that in Firefox on Mac (work laptop) and both Linux and Windows (personal laptop). Try adding the h.264 extension. That forces YouTube to provide h.264 videos which is hardware accelerated on pretty much any hardware.

[FlashBlaze]

Tried the addon. Still nothing. I have also tried clean installing Firefox with no addons, but same issue.

[behnamoh]

Adding that extension disables 4k video on YouTube.