/usr/lib/firefox/ is in firefox-locale-el 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-el@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/el/
/usr/lib/firefox/distribution/searchplugins/locale/el/amazon-en-GB.xml
/usr/lib/firefox/distribution/searchplugins/locale/el/bing.xml
/usr/lib/firefox/distribution/searchplugins/locale/el/duckduckgo.xml
/usr/lib/firefox/distribution/searchplugins/locale/el/eBay-en-GB.xml
/usr/lib/firefox/distribution/searchplugins/locale/el/google.xml
/usr/lib/firefox/distribution/searchplugins/locale/el/wikipedia-el.xml
/usr/lib/firefox/distribution/searchplugins/locale/el/yahoo.xml