The remote Debian host is missing a security-related update.
Richard Zhu and Huzaifa Sidhpurwala discovered that an out-of-bounds memory write when playing Vorbis media files could result in the execution of arbitrary code.
Upgrade the firefox-esr packages. For the oldstable distribution (jessie), these problems have been fixed in version 52.7.2esr-1~deb8u1. For the stable distribution (stretch), these problems have been fixed in version 52.7.2esr-1~deb9u1.