/usr/lib/ is in thunderbird-locale-be 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/be/
/usr/lib/thunderbird/distribution/searchplugins/locale/be/yandex.by.xml
/usr/lib/thunderbird/distribution/searchplugins/locale/be/be.wikipedia.org.xml
/usr/lib/thunderbird/distribution/searchplugins/locale/be/be-x-old.wikipedia.org.xml
/usr/lib/thunderbird/distribution/searchplugins/locale/be/ru.wikipedia.org-be.xml
/usr/lib/thunderbird/distribution/searchplugins/locale/be/google.xml
/usr/lib/thunderbird/distribution/searchplugins/locale/be/tut.by.xml
/usr/lib/thunderbird-addons/
/usr/lib/thunderbird-addons/extensions/
/usr/lib/thunderbird-addons/extensions/langpack-be@thunderbird.mozilla.org.xpi