Hi for me on Nvidia 10 series wayland plasma 6.
I also have the flickering, mostly in brave web apps like telegram.
Sometimes ungoogled chromium crypt.ee
I found that switching to the free nouveau driver fixes the issue for now.
Can be done permanently in kinoite/silverblue with:
sudo rpm-ostree kargs --delete-if-present=rd.driver.blacklist=nouveau --delete-if-present=modprobe.blacklist=nouveau --delete-if-present=nvidia-drm.modeset=1
to reverse:
sudo rpm-ostree kargs --append=rd.driver.blacklist=nouveau --append=modprobe.blacklist=nouveau --append=nvidia-drm.modeset=1
Hopefully explicit sync will fix this issue
Somewhat related, if you want good HW acceleration in Firefox (including video acceleration) see here and here for flatpaks.