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, spoofing, information disclosure, denial of service or cross-site request forgery. - CVE-2019-11719 and CVE-2019-11729 are only addressed for stretch, in buster Thunderbird uses the system-wide copy of NSS which will be updated separately.
Upgrade the thunderbird packages. For the oldstable distribution (stretch), these problems have been fixed in version 1:60.8.0-1~deb9u1. For the stable distribution (buster), these problems have been fixed in version 1:60.8.0-1~deb10u1.