/usr/lib/thunderbird/extensions/langpack-es-ar@lightning.debian.org is in lightning-l10n-es-ar 1:52.9.1-1~deb9u1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/thunderbird/extensions/../../../share/lightning/extensions/langpack-es-ar@lightning.debian.org