I have the same problem on ubuntu 21.04 wayland. Logging on under ubuntu xorg fixes it.
Windows sometimes don't close properly, and trying to exit can trigger a logoff. Manually logging off and back on fixes it for a while, until another window sticks.
Thunderbird version 1:78.11.0+build1-0ubuntu0.21.04.2
I have the same problem on ubuntu 21.04 wayland. Logging on under ubuntu xorg fixes it.
Windows sometimes don't close properly, and trying to exit can trigger a logoff. Manually logging off and back on fixes it for a while, until another window sticks.
Thunderbird version 1:78.11. 0+build1- 0ubuntu0. 21.04.2
Wayland is almost unusable because of this.