/usr/lib/ is in thunderbird-locale-it 1:24.4.0+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/it/
/usr/lib/thunderbird/distribution/searchplugins/locale/it/bing.xml
/usr/lib/thunderbird/distribution/searchplugins/locale/it/wikipedia-it.xml
/usr/lib/thunderbird/distribution/searchplugins/locale/it/eBay-it.xml
/usr/lib/thunderbird/distribution/searchplugins/locale/it/yahoo-it.xml
/usr/lib/thunderbird/distribution/searchplugins/locale/it/google.xml
/usr/lib/thunderbird/distribution/searchplugins/locale/it/twitter.xml
/usr/lib/thunderbird/distribution/searchplugins/locale/it/amazon-it.xml
/usr/lib/thunderbird-addons/
/usr/lib/thunderbird-addons/extensions/
/usr/lib/thunderbird-addons/extensions/langpack-it@thunderbird.mozilla.org.xpi