/usr/lib/ is in firefox-locale-sr 45.0.2+build1-0ubuntu1.
This file is owned by root:root, with mode 0o755.
..
/usr/lib/firefox/distribution/
/usr/lib/firefox/distribution/searchplugins/
/usr/lib/firefox/distribution/searchplugins/locale/
/usr/lib/firefox/distribution/searchplugins/locale/sr/
/usr/lib/firefox-addons/
/usr/lib/firefox-addons/extensions/
/usr/lib/firefox-addons/extensions/langpack-sr@firefox.mozilla.org.xpi
/usr/lib/firefox/
/usr/lib/firefox/distribution/searchplugins/locale/sr/ddg.xml
/usr/lib/firefox/distribution/searchplugins/locale/sr/amazon-en-GB.xml
/usr/lib/firefox/distribution/searchplugins/locale/sr/google.xml
/usr/lib/firefox/webapprt/
/usr/lib/firefox/webapprt/extensions/
/usr/lib/firefox/webapprt/extensions/langpack-sr@firefox.mozilla.org.xpi