/usr/lib/thunderbird/distribution/searchplugins/ is in thunderbird-locale-ru 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/ru/
/usr/lib/thunderbird/distribution/searchplugins/locale/ru/bing.xml
/usr/lib/thunderbird/distribution/searchplugins/locale/ru/duckduckgo-ru.xml
/usr/lib/thunderbird/distribution/searchplugins/locale/ru/google.xml
/usr/lib/thunderbird/distribution/searchplugins/locale/ru/ozonru.xml
/usr/lib/thunderbird/distribution/searchplugins/locale/ru/priceru.xml
/usr/lib/thunderbird/distribution/searchplugins/locale/ru/wikipedia-ru.xml
/usr/lib/thunderbird/distribution/searchplugins/locale/ru/yandex-slovari.xml
/usr/lib/thunderbird/distribution/searchplugins/locale/ru/yandex.xml