Comment 18 for bug 1850064

@GrzesiekC: I couldn't find any trace of a thunderbird apparmor profile in official Ubuntu packages (apart from the ubuntu-email abstraction). So I'm not sure where this profile you have enabled on your machine comes from, but not from the archive, that's for sure. I'd suggest disabling it:

    sudo apparmor_parser -R thunderbird

and create a symlink to the overly strict profile in /etc/apparmor.d/disable.