Thunderbird 78.3.1 is not very stable. With full wayland on KDE, the rendering is weird and flickering occurs. I managed to temporarily fix that by aliasing thunderbird to GDK_BACKEND=x11 thunderbird. Hopefully the situation can improve soon.
Full wayland is still not stable. It is a work in progress. Do you see this with X or Xwayland??
Thunderbird functions normally with X or Xwayland.
I know full wayland is not stable, but it is stable enough to me for daily use except that there are small bugs here and there. However, new version of thunderbird is unusable in full wayland, that’s why I pointed it out here and suggested a temporary fix 
Yes, this is quite well explained here…
https://wiki.archlinux.org/index.php/wayland#GUI_libraries
Similar can also be forced for QT apps which require use of an X11 session.