/usr/lib/ is in thunderbird-locale-sl 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/sl/
/usr/lib/thunderbird/distribution/searchplugins/locale/sl/najdi-si.xml
/usr/lib/thunderbird/distribution/searchplugins/locale/sl/wikipedia-sl.xml
/usr/lib/thunderbird/distribution/searchplugins/locale/sl/odpiralni.xml
/usr/lib/thunderbird/distribution/searchplugins/locale/sl/ceneji.xml
/usr/lib/thunderbird/distribution/searchplugins/locale/sl/google.xml
/usr/lib/thunderbird/distribution/searchplugins/locale/sl/twitter.xml
/usr/lib/thunderbird-addons/
/usr/lib/thunderbird-addons/extensions/
/usr/lib/thunderbird-addons/extensions/langpack-sl@thunderbird.mozilla.org.xpi