/usr/lib/firefox/distribution/searchplugins/ is in firefox-locale-mk 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/mk/
/usr/lib/firefox/distribution/searchplugins/locale/mk/amazondotcom.xml
/usr/lib/firefox/distribution/searchplugins/locale/mk/answers.xml
/usr/lib/firefox/distribution/searchplugins/locale/mk/bing.xml
/usr/lib/firefox/distribution/searchplugins/locale/mk/creativecommons.xml
/usr/lib/firefox/distribution/searchplugins/locale/mk/duckduckgo.xml
/usr/lib/firefox/distribution/searchplugins/locale/mk/eBay.xml
/usr/lib/firefox/distribution/searchplugins/locale/mk/google.xml
/usr/lib/firefox/distribution/searchplugins/locale/mk/wikipedia-mk.xml
/usr/lib/firefox/distribution/searchplugins/locale/mk/yahoo.xml