/usr/lib/ is in thunderbird-locale-dsb 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/dsb/
/usr/lib/thunderbird/distribution/searchplugins/locale/dsb/eBay.xml
/usr/lib/thunderbird/distribution/searchplugins/locale/dsb/yahoo.xml
/usr/lib/thunderbird/distribution/searchplugins/locale/dsb/twitter.xml
/usr/lib/thunderbird/distribution/searchplugins/locale/dsb/amazondotcom.xml
/usr/lib/thunderbird/distribution/searchplugins/locale/dsb/wikipedia.xml
/usr/lib/thunderbird/distribution/searchplugins/locale/dsb/bing.xml
/usr/lib/thunderbird/distribution/searchplugins/locale/dsb/aol-web-search.xml
/usr/lib/thunderbird-addons/
/usr/lib/thunderbird-addons/extensions/
/usr/lib/thunderbird-addons/extensions/langpack-dsb@thunderbird.mozilla.org.xpi