SUMMARY
Using Firefox with native Wayland (firefox-wayland, or Firefox with MOZ_ENABLE_WAYLAND=1 set) regularly results in flickering, issues with windows not taking up the required size, and ghosting when Firefox is terminated or exits.

STEPS TO REPRODUCE
1. Run "firefox-wayland" OR: MOZ_ENABLE_WAYLAND=1 firefox
2. Try various things such as resizing the window (maximising/minimising) or switching tabs, browsing the web etc.
3. Exit Firefox

OBSERVED RESULT
STEPS 1-2: Eventually you will see the window flickering and becoming unusable as artifacts appear all over the place.
STEP 3: There is a 50/50 chance that ghosted pieces of Firefox window will remain.

EXPECTED RESULT
Smooth, predictable operation, as expected with any application. Has been a continual problem even with 5.20.5 and earlier.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: Fedora Rawhide/Plasma 5.20.90
(available in About System)
KDE Plasma Version: 5.20.90
KDE Frameworks Version: 5.78.0
Qt Version: 5.15.2

ADDITIONAL INFORMATION