The remote Debian host is missing a security-related update.
Multiple security issues have been found in Thunderbird which could potentially result in the execution of arbitrary code, cross-site scripting, information disclosure and a covert content attack on S/MIME encryption using a crafted multipart/alternative message.
Upgrade the thunderbird packages. For the oldstable distribution (stretch), these problems have been fixed in version 1:60.9.0-1~deb9u1. For the stable distribution (buster), these problems have been fixed in version 1:60.9.0-1~deb10u1.