/usr/lib/firefox/distribution/searchplugins/ is in firefox-locale-sk 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/sk/
/usr/lib/firefox/distribution/searchplugins/locale/sk/atlas-sk.xml
/usr/lib/firefox/distribution/searchplugins/locale/sk/azet-sk.xml
/usr/lib/firefox/distribution/searchplugins/locale/sk/duckduckgo.xml
/usr/lib/firefox/distribution/searchplugins/locale/sk/dunaj-sk.xml
/usr/lib/firefox/distribution/searchplugins/locale/sk/eBay.xml
/usr/lib/firefox/distribution/searchplugins/locale/sk/google.xml
/usr/lib/firefox/distribution/searchplugins/locale/sk/slovnik-sk.xml
/usr/lib/firefox/distribution/searchplugins/locale/sk/wikipedia-sk.xml
/usr/lib/firefox/distribution/searchplugins/locale/sk/zoznam-sk.xml