/usr/lib/thunderbird/distribution/searchplugins/ is in thunderbird-locale-es 1:38.6.0+build1-0ubuntu1.
This file is owned by root:root, with mode 0o755.
..
/usr/lib/thunderbird/distribution/searchplugins/locale/
/usr/lib/thunderbird/distribution/searchplugins/locale/es-AR/
/usr/lib/thunderbird/distribution/searchplugins/locale/es-AR/amazondotcom.xml
/usr/lib/thunderbird/distribution/searchplugins/locale/es-AR/drae.xml
/usr/lib/thunderbird/distribution/searchplugins/locale/es-AR/google.xml
/usr/lib/thunderbird/distribution/searchplugins/locale/es-AR/mercadolibre-ar.xml
/usr/lib/thunderbird/distribution/searchplugins/locale/es-AR/wikipedia-es.xml
/usr/lib/thunderbird/distribution/searchplugins/locale/es-AR/yahoo-ar.xml
/usr/lib/thunderbird/distribution/searchplugins/locale/es-ES/
/usr/lib/thunderbird/distribution/searchplugins/locale/es-ES/bing.xml
/usr/lib/thunderbird/distribution/searchplugins/locale/es-ES/drae.xml
/usr/lib/thunderbird/distribution/searchplugins/locale/es-ES/eBay-es.xml
/usr/lib/thunderbird/distribution/searchplugins/locale/es-ES/google.xml
/usr/lib/thunderbird/distribution/searchplugins/locale/es-ES/twitter.xml
/usr/lib/thunderbird/distribution/searchplugins/locale/es-ES/wikipedia-es.xml
/usr/lib/thunderbird/distribution/searchplugins/locale/es-ES/yahoo-es.xml