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