/usr/lib/ is in firefox-locale-sl 11.0+build1-0ubuntu4.
This file is owned by root:root, with mode 0o755.
..
/usr/lib/firefox-addons/
/usr/lib/firefox-addons/extensions/
/usr/lib/firefox-addons/extensions/langpack-sl@firefox.mozilla.org.xpi
/usr/lib/firefox/
/usr/lib/firefox/distribution/
/usr/lib/firefox/distribution/searchplugins/
/usr/lib/firefox/distribution/searchplugins/locale/
/usr/lib/firefox/distribution/searchplugins/locale/sl/
/usr/lib/firefox/distribution/searchplugins/locale/sl/ceneji.xml
/usr/lib/firefox/distribution/searchplugins/locale/sl/google.xml
/usr/lib/firefox/distribution/searchplugins/locale/sl/najdi-si.xml
/usr/lib/firefox/distribution/searchplugins/locale/sl/odpiralni.xml
/usr/lib/firefox/distribution/searchplugins/locale/sl/twitter.xml
/usr/lib/firefox/distribution/searchplugins/locale/sl/wikipedia-sl.xml