I was experiencing a significant amount of dropped frames during video playback after switching to Wayland and finally was able to find a fix that worked for me
During youtube playback I would experience 10+ frames dropped all at once, causing a noticable skip.
Saw a few people having similar issues:
https://forum.manjaro.org/t/my-browser-drops-a-bunch-of-frames-while-playing-youtube-videos/168839
Some system info in case it's hardware/software specific:
Distro: KDE Neon
DE: KDE 6.2
Compositor: KWin
GPU: AMD Radeon 5500XT
Kernel: 6.8.0-49-generic
Fix that worked for me:
Tweaking about:config settings:
media.ffmpeg.vaapi.enabled=true
media.hardware-video-decoding.force-enabled=true
widget.wayland.opaque-region.enabled=false
widget.wayland.vsync.enabled=false
I'm pretty sure it's all related to "widget.wayland.vsync.enabled" being set to false, but it's nice to have hardware acceleration too.
Other notes:
This issue seems to be more pronounced when you have a lot of tabs and extensions. I have over 3000 tabs (I have a problem, I know) and various extensions.
To test things out I created a brand new profile. With just a single tab I noticed no noticeable frame drops
Once I got to 1000+ tabs in the new profile, the frame drops became more pronounced and noticeable due to occasionally dropping multiple frames at once.
It never got as bad as my main profile but I also never installed extensions either.
Anyways I just wanted to document the issue. If I get time I'll try to troubleshoot further and may try to open a firefox bug. Since this does seem related to the number of tabs though I'm not sure if it'll be high priority.
Wanted to at least create a gist in hopes someone searches for it and hopefully this answer helps.