/usr/lib/thunderbird/ is in thunderbird-locale-fr 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-fr@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/fr/
/usr/lib/thunderbird/distribution/searchplugins/locale/fr/amazon-france.xml
/usr/lib/thunderbird/distribution/searchplugins/locale/fr/bing.xml
/usr/lib/thunderbird/distribution/searchplugins/locale/fr/cnrtl-tlfi-fr.xml
/usr/lib/thunderbird/distribution/searchplugins/locale/fr/eBay-france.xml
/usr/lib/thunderbird/distribution/searchplugins/locale/fr/wikipedia-fr.xml
/usr/lib/thunderbird/distribution/searchplugins/locale/fr/yahoo-france.xml