/usr/lib/firefox/distribution/searchplugins/ is in firefox-locale-en 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/en-GB/
/usr/lib/firefox/distribution/searchplugins/locale/en-GB/amazon-en-GB.xml
/usr/lib/firefox/distribution/searchplugins/locale/en-GB/bing.xml
/usr/lib/firefox/distribution/searchplugins/locale/en-GB/chambers-en-GB.xml
/usr/lib/firefox/distribution/searchplugins/locale/en-GB/duckduckgo.xml
/usr/lib/firefox/distribution/searchplugins/locale/en-GB/eBay-en-GB.xml
/usr/lib/firefox/distribution/searchplugins/locale/en-GB/google.xml
/usr/lib/firefox/distribution/searchplugins/locale/en-GB/twitter.xml
/usr/lib/firefox/distribution/searchplugins/locale/en-GB/wikipedia.xml
/usr/lib/firefox/distribution/searchplugins/locale/en-GB/yahoo-en-GB.xml
/usr/lib/firefox/distribution/searchplugins/locale/en-ZA/
/usr/lib/firefox/distribution/searchplugins/locale/en-ZA/amazondotcom.xml
/usr/lib/firefox/distribution/searchplugins/locale/en-ZA/bing.xml
/usr/lib/firefox/distribution/searchplugins/locale/en-ZA/duckduckgo.xml
/usr/lib/firefox/distribution/searchplugins/locale/en-ZA/google.xml
/usr/lib/firefox/distribution/searchplugins/locale/en-ZA/twitter.xml
/usr/lib/firefox/distribution/searchplugins/locale/en-ZA/wikipedia.xml