/usr/lib/ is in thunderbird-locale-rm 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/rm/
/usr/lib/thunderbird/distribution/searchplugins/locale/rm/pledarigrond.xml
/usr/lib/thunderbird/distribution/searchplugins/locale/rm/yahoo-ch.xml
/usr/lib/thunderbird/distribution/searchplugins/locale/rm/leo_ende_de.xml
/usr/lib/thunderbird/distribution/searchplugins/locale/rm/wikipedia-rm.xml
/usr/lib/thunderbird/distribution/searchplugins/locale/rm/bing.xml
/usr/lib/thunderbird/distribution/searchplugins/locale/rm/google.xml
/usr/lib/thunderbird-addons/
/usr/lib/thunderbird-addons/extensions/
/usr/lib/thunderbird-addons/extensions/langpack-rm@thunderbird.mozilla.org.xpi