/usr/lib/thunderbird-11.0.1/plugins 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/../thunderbird-addons/plugins
/usr/lib/thunderbird-11.0.1/../thunderbird-addons/plugins