User Agent: Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:83.0) Gecko/20100101 Firefox/83.0

Steps to reproduce:

On a Nvidia Optimus laptop featuring intel UHD Graphics 630 and 1650ti mobile, iGPU does not decode video playback in Youtube and likely other sites as of Firefox 82. Tested Firefox 81 and 68 ESR and works without issue. Tested on 82, 83, and nightly 85, video playback becomes choppy above 30fps. Changing webrender settings in about:config to gfx.webrender.enable=true gfx.webrender.all=true and gfx.webrender.compositer=true and changing media.ffmpeg.vaapi.enable=true media.ffvpx.enable=false provides better (though not perfect) playback. No adjustments need to be made in Firefox 81 and below and they provide smoother playback, even with 82+ having made the correct adjustments

Actual results:

Video playback up to 4k 30 seems smooth, but all higher framerate video including 720p 60 induces significant stutter and choppiness if rendering in window that size or larger

Expected results:

Video playback should be smooth on iGPU at least up to 4k 60 in my testing with Firefox 81 and 68.

Adding additional detail, this affects all major Linux distributions as I've tested on several flavors of Ubuntu, Manjaro, and Fedora. Have not tested Windows or MacOS, though MacOS does not support this laptop (duh).

One more major detail. This issue appears to occur worse on a device I have running Intel HD 630 w/out a dedicated GPU. Video playback completely freezes on that device. Older Firefox versions do not.

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