/usr/lib/firefox/distribution/searchplugins/ is in firefox-locale-eu 28.0+build2-0ubuntu2.
This file is owned by root:root, with mode 0o755.
..
/usr/lib/firefox/distribution/searchplugins/locale/
/usr/lib/firefox/distribution/searchplugins/locale/eu/
/usr/lib/firefox/distribution/searchplugins/locale/eu/amazon-en-GB.xml
/usr/lib/firefox/distribution/searchplugins/locale/eu/bing.xml
/usr/lib/firefox/distribution/searchplugins/locale/eu/creativecommons.xml
/usr/lib/firefox/distribution/searchplugins/locale/eu/duckduckgo.xml
/usr/lib/firefox/distribution/searchplugins/locale/eu/eBay-es.xml
/usr/lib/firefox/distribution/searchplugins/locale/eu/elebila.xml
/usr/lib/firefox/distribution/searchplugins/locale/eu/google.xml
/usr/lib/firefox/distribution/searchplugins/locale/eu/wikipedia-eu.xml
/usr/lib/firefox/distribution/searchplugins/locale/eu/yahoo.xml