/usr/lib/firefox/distribution/searchplugins/ is in firefox 28.0+build2-0ubuntu2.
This file is owned by root:root, with mode 0o755.
..
/usr/lib/firefox/distribution/searchplugins/common/
/usr/lib/firefox/distribution/searchplugins/locale/
/usr/lib/firefox/distribution/searchplugins/locale/en-US/
/usr/lib/firefox/distribution/searchplugins/locale/en-US/amazondotcom.xml
/usr/lib/firefox/distribution/searchplugins/locale/en-US/bing.xml
/usr/lib/firefox/distribution/searchplugins/locale/en-US/duckduckgo.xml
/usr/lib/firefox/distribution/searchplugins/locale/en-US/eBay.xml
/usr/lib/firefox/distribution/searchplugins/locale/en-US/google.xml
/usr/lib/firefox/distribution/searchplugins/locale/en-US/twitter.xml
/usr/lib/firefox/distribution/searchplugins/locale/en-US/wikipedia.xml
/usr/lib/firefox/distribution/searchplugins/locale/en-US/yahoo.xml