You can try vaapi acceleration on latest Firefox too on linux.
On Firefox stable go to about:config and set :
gfx.x11-egl.force-enabled to true media.ffmpeg.vaapi-drm-display.enabled to true media.ffmpeg.vaapi.enabled to true
media.ffvpx.enabled to false
Then install firefox add "h264ify" for youtube. Then play some video and watch the cpu usage got drop or still high.
And add addon "h264ify-embed-fix" for hardware acceleration other than youtube website eg vimeo.
Firefox getting better and better with their latest release. Cant wait for "WebGpu" to be implement on firefox stable.