The remote Debian host is missing a security-related update.
Several security issues have been found in the Mozilla Firefox web browser: Multiple memory safety errors, use-after-frees, integer overflows and other implementation errors may lead to the execution of arbitrary code, denial of service or URL spoofing.
Upgrade the firefox-esr packages. For the oldstable distribution (jessie), these problems have been fixed in version 52.6.0esr-1~deb8u1. For the stable distribution (stretch), these problems have been fixed in version 52.6.0esr-1~deb9u1.