I was testing another (crash on launch) bug and a Mozilla developer told me to install nightly. Crash on launch stopped happening so I thought I better keep it for testing.
I did a zypper dup just to make sure system is up to date and I just have "wayland (full)" enabled on this KDE 5.20 installation. I enabled ffmpeg.vaapi but it doesn't seem to be related. Even webrender.force.disabled didn't make a change. In 12 hours, I had chance to check 2 versions. No change.
When you enter youtube.com or any site featuring video, it doesn't play. I tested the vaapi acceleration using mpv and vainfo (all details on firefox bug report).
They rightfully suggest this could be a TW only issue. So I thought I better alert.
Bugzilla link: https://bugzilla.mozilla.org/show_bug.cgi?id=1671652
Intel GPU here (hd 5500)