I'm answering my own question: I have managed to reproduce the issue with the upstream build for 78.13.0, downloaded from thunderbird.net, in a fully up-to-date Ubuntu 21.04 VM, executed with MOZ_ENABLE_WAYLAND=1.
I'm answering my own question: I have managed to reproduce the issue with the upstream build for 78.13.0, downloaded from thunderbird.net, in a fully up-to-date Ubuntu 21.04 VM, executed with MOZ_ENABLE_ WAYLAND= 1.