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