/usr/lib/firefox/distribution/searchplugins/ is in firefox-locale-zh-hant 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/zh-TW/
/usr/lib/firefox/distribution/searchplugins/locale/zh-TW/creativecommons.xml
/usr/lib/firefox/distribution/searchplugins/locale/zh-TW/duckduckgo.xml
/usr/lib/firefox/distribution/searchplugins/locale/zh-TW/findbook-zh-TW.xml
/usr/lib/firefox/distribution/searchplugins/locale/zh-TW/google.xml
/usr/lib/firefox/distribution/searchplugins/locale/zh-TW/wikipedia-zh-TW.xml
/usr/lib/firefox/distribution/searchplugins/locale/zh-TW/yahoo-answer-zh-TW.xml
/usr/lib/firefox/distribution/searchplugins/locale/zh-TW/yahoo-bid-zh-TW.xml
/usr/lib/firefox/distribution/searchplugins/locale/zh-TW/yahoo-zh-TW.xml