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Elementary OS 6 Odin Thunderbird 91.2.1 (64-bit) flatpak - 1.0
All is working fine apart from Local folders occasionally not connecting (they are located on NAS, which connects at boot perfectly fine) It is a simple matter of editing the settings every time it happens (Thunderbird re-starts and all is fine)but...
it was all (and still is) working fine in EOS 5 , so why is it not working in EOS 6 ??
It seems that the issue is caused by Flatpak.
un-installing Flatpak version and replacing it with:
sudo apt install Thunderbird
solves the problem.
It appears to be common problem for Mozilla apps, see also: