/usr/lib/icedove/extensions/langpack-pl@thunderbird.mozilla.org.xpi is in icedove-l10n-pl 1:38.0.1-1~deb7u1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/icedove/extensions/../../../share/icedove/extensions/langpack-pl@thunderbird.mozilla.org.xpi