/usr/lib/thunderbird-11.0.1/dictionaries is in thunderbird 11.0.1+build1-0ubuntu2.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/thunderbird-11.0.1/../../share/hunspell
/usr/lib/thunderbird-11.0.1/../../share/hunspell