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, information disclosure or spoofing of sender's email addresses.
Upgrade the thunderbird packages. For the oldstable distribution (jessie), these problems have been fixed in version 1:52.5.2-2~deb8u1. For the stable distribution (stretch), these problems have been fixed in version 1:52.5.2-2~deb9u1.