/usr/lib/firefox/distribution/ is in firefox-locale-cy 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/cy/
/usr/lib/firefox/distribution/searchplugins/locale/cy/amazon-en-GB.xml
/usr/lib/firefox/distribution/searchplugins/locale/cy/creativecommons.xml
/usr/lib/firefox/distribution/searchplugins/locale/cy/duckduckgo.xml
/usr/lib/firefox/distribution/searchplugins/locale/cy/eBay-en-GB.xml
/usr/lib/firefox/distribution/searchplugins/locale/cy/google.xml
/usr/lib/firefox/distribution/searchplugins/locale/cy/palasprint.xml
/usr/lib/firefox/distribution/searchplugins/locale/cy/termau.xml
/usr/lib/firefox/distribution/searchplugins/locale/cy/wikipedia-cy.xml
/usr/lib/firefox/distribution/searchplugins/locale/cy/yahoo-en-GB.xml