/usr/lib/ is in thunderbird-locale-et 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/et/
/usr/lib/thunderbird/distribution/searchplugins/locale/et/eBay.xml
/usr/lib/thunderbird/distribution/searchplugins/locale/et/eki-ee.xml
/usr/lib/thunderbird/distribution/searchplugins/locale/et/wikipedia-et.xml
/usr/lib/thunderbird/distribution/searchplugins/locale/et/neti-ee.xml
/usr/lib/thunderbird/distribution/searchplugins/locale/et/osta-ee.xml
/usr/lib/thunderbird/distribution/searchplugins/locale/et/google.xml
/usr/lib/thunderbird-addons/
/usr/lib/thunderbird-addons/extensions/
/usr/lib/thunderbird-addons/extensions/langpack-et@thunderbird.mozilla.org.xpi