/usr/lib/thunderbird/distribution/searchplugins/ is in thunderbird-locale-ja 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/ja/
/usr/lib/thunderbird/distribution/searchplugins/locale/ja/amazon-jp.xml
/usr/lib/thunderbird/distribution/searchplugins/locale/ja/google-jp.xml
/usr/lib/thunderbird/distribution/searchplugins/locale/ja/oshiete-goo.xml
/usr/lib/thunderbird/distribution/searchplugins/locale/ja/rakuten.xml
/usr/lib/thunderbird/distribution/searchplugins/locale/ja/twitter-ja.xml
/usr/lib/thunderbird/distribution/searchplugins/locale/ja/wikipedia-ja.xml
/usr/lib/thunderbird/distribution/searchplugins/locale/ja/yahoo-jp-auctions.xml
/usr/lib/thunderbird/distribution/searchplugins/locale/ja/yahoo-jp.xml