It's in fact not plenty fast. My PC heats up considerably watching 4k youtube videos. And in fact it stutters with 4k 60fps. How is that acceptable? How is it acceptable I cannot use one of the most popular websites optimally. And that's not even talking about all the wasted energy.
Where do you find 4k youtube videos? The best I ever encounter are 1080p. Often enough the best I can find are 720p or even worse. Not that it usually matters.
I use mpv when I am serious, moving FF entirely out of the picture.
Here are a few channels that I watch ofen that always upload in 4k. This is not a niche for serious youtube channels:
https://www.youtube.com/channel/UCsn6cjffsvyOZCZxvGoJxGg
https://www.youtube.com/channel/UCHnyfMqiRRG1u-2MsSQLbXA
https://www.youtube.com/channel/UCSpFnDQr88xCZ80N-X7t0nQ
https://www.youtube.com/channel/UCXuqSBlHAE6Xw-yeJA0Tunw
Besides even at 1080p the difference in how much battery it saves is astounding. With acceleration your CPU is just idling while without it has to work quite hard.
If they have a setup that can display and content that they want to watch in 4K without their laptop burning a mark on their lap, then that's a valid use case. An use case that doesn't work with Firefox at the moment because of Firefox. "You're holding it wrong" is an incredibly inane attitude.
I can't see what benefit you expect to get from the GPU. It is plenty fast with all-CPU rendering.
The Qubes X model is interesting: each VM runs its own X, and renders windows into memory shared with a central VM that copies the pixels to real video memory. Input events get forwarded to a VM's X according to which is the active window frame. The camera can be attached temporarily to a VM, and mike input similarly. So, no VM has physical access to hardware anytime other than when you want it to.
I am hoping a future Qubes will have proxying Vulkan stubs or something, for controlled access to a GPU. But it isn't missed much since FF stopped crashing.