/usr/lib/thunderbird/extensions/langpack-sl@thunderbird.mozilla.org.xpi is in thunderbird-l10n-sl 1:52.8.0-1~deb8u1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/thunderbird/extensions/../../../share/thunderbird/extensions/langpack-sl@thunderbird.mozilla.org.xpi