/usr/lib/ is in thunderbird-locale-nn 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/nn-NO/
/usr/lib/thunderbird/distribution/searchplugins/locale/nn-NO/gulesider-NO.xml
/usr/lib/thunderbird/distribution/searchplugins/locale/nn-NO/eBay.xml
/usr/lib/thunderbird/distribution/searchplugins/locale/nn-NO/yahoo.xml
/usr/lib/thunderbird/distribution/searchplugins/locale/nn-NO/twitter.xml
/usr/lib/thunderbird/distribution/searchplugins/locale/nn-NO/amazondotcom.xml
/usr/lib/thunderbird/distribution/searchplugins/locale/nn-NO/wikipedia.xml
/usr/lib/thunderbird/distribution/searchplugins/locale/nn-NO/bing.xml
/usr/lib/thunderbird/distribution/searchplugins/locale/nn-NO/aol-web-search.xml
/usr/lib/thunderbird-addons/
/usr/lib/thunderbird-addons/extensions/
/usr/lib/thunderbird-addons/extensions/langpack-nn-NO@thunderbird.mozilla.org.xpi