/usr/lib/thunderbird/ is in thunderbird-locale-sr 1:24.4.0+build1-0ubuntu1.
This file is owned by root:root, with mode 0o755.
..
/usr/lib/thunderbird-addons/extensions/
/usr/lib/thunderbird-addons/extensions/langpack-sr@thunderbird.mozilla.org.xpi
/usr/lib/thunderbird/distribution/
/usr/lib/thunderbird/distribution/searchplugins/
/usr/lib/thunderbird/distribution/searchplugins/locale/
/usr/lib/thunderbird/distribution/searchplugins/locale/sr/
/usr/lib/thunderbird/distribution/searchplugins/locale/sr/amazondotcom.xml
/usr/lib/thunderbird/distribution/searchplugins/locale/sr/aol-web-search.xml
/usr/lib/thunderbird/distribution/searchplugins/locale/sr/bing.xml
/usr/lib/thunderbird/distribution/searchplugins/locale/sr/eBay.xml
/usr/lib/thunderbird/distribution/searchplugins/locale/sr/google.xml
/usr/lib/thunderbird/distribution/searchplugins/locale/sr/twitter.xml
/usr/lib/thunderbird/distribution/searchplugins/locale/sr/wikipedia.xml
/usr/lib/thunderbird/distribution/searchplugins/locale/sr/yahoo.xml