Ubuntu Gnome 21.04 Software Update breaks Thunderbird window management
It appears to be a bug in the GNOME Shell.
Switching to X.org (instead of Wayland) makes the problem go away. Simply select the X.org session when entering password on the login screen.
You can still use Wayland with its advantages for the Gnome session but start Thunderbird in X11 mode. See https://bugs.launchpad.net/ubuntu/+source/thunderbird/+bug/1932328 and https://askubuntu.com/a/1355533/447487