The remote Debian host is missing a security-related update.
It was discovered that a use-after-free in the compositor of Firefox can result in the execution of arbitrary code.
Upgrade the firefox-esr packages. For the oldstable distribution (jessie), this problem has been fixed in version 52.7.3esr-1~deb8u1. For the stable distribution (stretch), this problem has been fixed in version 52.7.3esr-1~deb9u1.