/usr/lib/firefox/distribution/searchplugins/ is in firefox-locale-ga 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/ga-IE/
/usr/lib/firefox/distribution/searchplugins/locale/ga-IE/amazon-en-GB.xml
/usr/lib/firefox/distribution/searchplugins/locale/ga-IE/answers.xml
/usr/lib/firefox/distribution/searchplugins/locale/ga-IE/creativecommons.xml
/usr/lib/firefox/distribution/searchplugins/locale/ga-IE/duckduckgo.xml
/usr/lib/firefox/distribution/searchplugins/locale/ga-IE/eBay-en-GB.xml
/usr/lib/firefox/distribution/searchplugins/locale/ga-IE/focal.xml
/usr/lib/firefox/distribution/searchplugins/locale/ga-IE/google.xml
/usr/lib/firefox/distribution/searchplugins/locale/ga-IE/twitter.xml
/usr/lib/firefox/distribution/searchplugins/locale/ga-IE/wikipedia-ga-IE.xml
/usr/lib/firefox/distribution/searchplugins/locale/ga-IE/yahoo-en-GB.xml