/usr/lib/thunderbird/ is in thunderbird-locale-cs 1:38.6.0+build1-0ubuntu1.
This file is owned by root:root, with mode 0o755.
..
/usr/lib/thunderbird-addons/extensions/
/usr/lib/thunderbird-addons/extensions/langpack-cs@thunderbird.mozilla.org.xpi
/usr/lib/thunderbird/distribution/
/usr/lib/thunderbird/distribution/searchplugins/
/usr/lib/thunderbird/distribution/searchplugins/locale/
/usr/lib/thunderbird/distribution/searchplugins/locale/cs/
/usr/lib/thunderbird/distribution/searchplugins/locale/cs/bing.xml
/usr/lib/thunderbird/distribution/searchplugins/locale/cs/google.xml
/usr/lib/thunderbird/distribution/searchplugins/locale/cs/heureka-cz.xml
/usr/lib/thunderbird/distribution/searchplugins/locale/cs/mapy-cz.xml
/usr/lib/thunderbird/distribution/searchplugins/locale/cs/seznam-cz.xml
/usr/lib/thunderbird/distribution/searchplugins/locale/cs/slunecnice-cz.xml
/usr/lib/thunderbird/distribution/searchplugins/locale/cs/twitter.xml
/usr/lib/thunderbird/distribution/searchplugins/locale/cs/wikipedia-cz.xml