Comment 76 for bug 1816497

This used to do the trick for me. Since an upgrade to 20.04 the decoding is not working properly. Chromium (86 here) is not calling MojoVideoDecoder.
https://www.linuxuprising.com/2018/08/how-to-enable-hardware-accelerated.html

That may be related to a switch to the iHD driver (active now according to vainfo). I noticed other problems with ffpmeg for screengrabbing.