The remote Debian host is missing a security-related update.
Ivan Fratric discovered a buffer overflow in the Skia graphics library used by Firefox, which could result in the execution of arbitrary code.
Upgrade the firefox-esr packages. For the oldstable distribution (jessie), this problem has been fixed in version 52.8.1esr-1~deb8u1. For the stable distribution (stretch), this problem has been fixed in version 52.8.1esr-1~deb9u1.