The remote Debian host is missing a security-related update.
Multiple security issues have been found in Thunderbird, which may lead to the execution of arbitrary code, denial of service or attacks on encrypted emails.
Upgrade the thunderbird packages. For the oldstable distribution (jessie), these problems have been fixed in version 1:52.8.0-1~deb8u1. For the stable distribution (stretch), these problems have been fixed in version 1:52.8.0-1~deb9u1.