/usr/lib/thunderbird/ is in thunderbird-locale-fy 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-fy-NL@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/fy-NL/
/usr/lib/thunderbird/distribution/searchplugins/locale/fy-NL/amazondotcom.xml
/usr/lib/thunderbird/distribution/searchplugins/locale/fy-NL/aol-web-search.xml
/usr/lib/thunderbird/distribution/searchplugins/locale/fy-NL/bing.xml
/usr/lib/thunderbird/distribution/searchplugins/locale/fy-NL/eBay.xml
/usr/lib/thunderbird/distribution/searchplugins/locale/fy-NL/twitter.xml
/usr/lib/thunderbird/distribution/searchplugins/locale/fy-NL/wikipedia.xml
/usr/lib/thunderbird/distribution/searchplugins/locale/fy-NL/yahoo.xml