/usr/lib/ is in thunderbird-locale-gd 1:38.6.0+build1-0ubuntu1.
This file is owned by root:root, with mode 0o755.
..
/usr/lib/thunderbird/
/usr/lib/thunderbird/distribution/
/usr/lib/thunderbird/distribution/searchplugins/
/usr/lib/thunderbird/distribution/searchplugins/locale/
/usr/lib/thunderbird/distribution/searchplugins/locale/gd/
/usr/lib/thunderbird/distribution/searchplugins/locale/gd/wikipedia-gd.xml
/usr/lib/thunderbird/distribution/searchplugins/locale/gd/yahoo-en-GB.xml
/usr/lib/thunderbird/distribution/searchplugins/locale/gd/amazon-en-GB.xml
/usr/lib/thunderbird/distribution/searchplugins/locale/gd/bbc-alba.xml
/usr/lib/thunderbird/distribution/searchplugins/locale/gd/faclair-beag.xml
/usr/lib/thunderbird/distribution/searchplugins/locale/gd/google.xml
/usr/lib/thunderbird-addons/
/usr/lib/thunderbird-addons/extensions/
/usr/lib/thunderbird-addons/extensions/langpack-gd@thunderbird.mozilla.org.xpi