/usr/lib/firefox/distribution/searchplugins/ is in firefox-locale-es 59.0.2+build1-0ubuntu1.
This file is owned by root:root, with mode 0o755.
..
/usr/lib/firefox/distribution/searchplugins/locale/
/usr/lib/firefox/distribution/searchplugins/locale/es-AR/
/usr/lib/firefox/distribution/searchplugins/locale/es-AR/amazondotcom.xml
/usr/lib/firefox/distribution/searchplugins/locale/es-AR/ddg.xml
/usr/lib/firefox/distribution/searchplugins/locale/es-AR/google.xml
/usr/lib/firefox/distribution/searchplugins/locale/es-CL/
/usr/lib/firefox/distribution/searchplugins/locale/es-CL/ddg.xml
/usr/lib/firefox/distribution/searchplugins/locale/es-CL/google.xml
/usr/lib/firefox/distribution/searchplugins/locale/es-ES/
/usr/lib/firefox/distribution/searchplugins/locale/es-ES/ddg.xml
/usr/lib/firefox/distribution/searchplugins/locale/es-ES/google.xml
/usr/lib/firefox/distribution/searchplugins/locale/es-MX/
/usr/lib/firefox/distribution/searchplugins/locale/es-MX/ddg.xml
/usr/lib/firefox/distribution/searchplugins/locale/es-MX/google.xml