/usr/lib/ is in firefox-locale-bg 45.0.2+build1-0ubuntu1.
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-bg@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/bg/
/usr/lib/firefox/distribution/searchplugins/locale/bg/amazondotcom.xml
/usr/lib/firefox/distribution/searchplugins/locale/bg/ddg.xml
/usr/lib/firefox/distribution/searchplugins/locale/bg/google.xml
/usr/lib/firefox/webapprt/
/usr/lib/firefox/webapprt/extensions/
/usr/lib/firefox/webapprt/extensions/langpack-bg@firefox.mozilla.org.xpi