/usr/lib/thunderbird/ is in thunderbird-locale-is 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-is@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/is/
/usr/lib/thunderbird/distribution/searchplugins/locale/is/amazondotcom.xml
/usr/lib/thunderbird/distribution/searchplugins/locale/is/bing.xml
/usr/lib/thunderbird/distribution/searchplugins/locale/is/eBay.xml
/usr/lib/thunderbird/distribution/searchplugins/locale/is/google.xml
/usr/lib/thunderbird/distribution/searchplugins/locale/is/leit-is.xml
/usr/lib/thunderbird/distribution/searchplugins/locale/is/wikipedia-is.xml
/usr/lib/thunderbird/distribution/searchplugins/locale/is/yahoo.xml