/usr/lib/firefox/distribution/searchplugins/ is in firefox-locale-pt 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/pt-BR/
/usr/lib/firefox/distribution/searchplugins/locale/pt-BR/bing.xml
/usr/lib/firefox/distribution/searchplugins/locale/pt-BR/buscape.xml
/usr/lib/firefox/distribution/searchplugins/locale/pt-BR/duckduckgo.xml
/usr/lib/firefox/distribution/searchplugins/locale/pt-BR/google.xml
/usr/lib/firefox/distribution/searchplugins/locale/pt-BR/mercadolivre.xml
/usr/lib/firefox/distribution/searchplugins/locale/pt-BR/twitter.xml
/usr/lib/firefox/distribution/searchplugins/locale/pt-BR/wikipedia-br.xml
/usr/lib/firefox/distribution/searchplugins/locale/pt-BR/yahoo-br.xml
/usr/lib/firefox/distribution/searchplugins/locale/pt-PT/
/usr/lib/firefox/distribution/searchplugins/locale/pt-PT/amazon-en-GB.xml
/usr/lib/firefox/distribution/searchplugins/locale/pt-PT/duckduckgo.xml
/usr/lib/firefox/distribution/searchplugins/locale/pt-PT/google.xml
/usr/lib/firefox/distribution/searchplugins/locale/pt-PT/priberam.xml
/usr/lib/firefox/distribution/searchplugins/locale/pt-PT/sapo.xml
/usr/lib/firefox/distribution/searchplugins/locale/pt-PT/wikipedia-ptpt.xml