/usr/lib/firefox/ is in firefox-locale-fy 28.0+build2-0ubuntu2.
This file is owned by root:root, with mode 0o755.
..
/usr/lib/firefox-addons/extensions/
/usr/lib/firefox-addons/extensions/langpack-fy-NL@firefox.mozilla.org.xpi
/usr/lib/firefox/distribution/
/usr/lib/firefox/distribution/searchplugins/
/usr/lib/firefox/distribution/searchplugins/locale/
/usr/lib/firefox/distribution/searchplugins/locale/fy-NL/
/usr/lib/firefox/distribution/searchplugins/locale/fy-NL/bing.xml
/usr/lib/firefox/distribution/searchplugins/locale/fy-NL/bolcom-fy-NL.xml
/usr/lib/firefox/distribution/searchplugins/locale/fy-NL/duckduckgo.xml
/usr/lib/firefox/distribution/searchplugins/locale/fy-NL/google.xml
/usr/lib/firefox/distribution/searchplugins/locale/fy-NL/marktplaats-fy-NL.xml
/usr/lib/firefox/distribution/searchplugins/locale/fy-NL/wikipedia-fy-NL.xml
/usr/lib/firefox/distribution/searchplugins/locale/fy-NL/yahoo-fy-NL.xml