/usr/lib/firefox/distribution/ is in firefox-locale-th 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/th/
/usr/lib/firefox/distribution/searchplugins/locale/th/amazondotcom.xml
/usr/lib/firefox/distribution/searchplugins/locale/th/bing.xml
/usr/lib/firefox/distribution/searchplugins/locale/th/creativecommons.xml
/usr/lib/firefox/distribution/searchplugins/locale/th/duckduckgo.xml
/usr/lib/firefox/distribution/searchplugins/locale/th/eBay.xml
/usr/lib/firefox/distribution/searchplugins/locale/th/google.xml
/usr/lib/firefox/distribution/searchplugins/locale/th/longdo.xml
/usr/lib/firefox/distribution/searchplugins/locale/th/wikipedia-th.xml
/usr/lib/firefox/distribution/searchplugins/locale/th/yahoo.xml