Memory safety bugs were reported in Firefox 56 and Firefox ESR 52.4. Some of these bugs showed evidence of memory corruption and we presume that with enough effort that some of these could be exploited to run arbitrary code. This vulnerability affects Firefox < 57, Firefox ESR < 52.5, and Thunderbird < 52.5.
https://www.mozilla.org/security/advisories/mfsa2017-26/
https://www.mozilla.org/security/advisories/mfsa2017-25/
https://www.mozilla.org/security/advisories/mfsa2017-24/
https://www.debian.org/security/2017/dsa-4075
https://www.debian.org/security/2017/dsa-4061
https://www.debian.org/security/2017/dsa-4035
https://usn.ubuntu.com/3688-1/
https://lists.debian.org/debian-lts-announce/2017/12/msg00001.html
https://lists.debian.org/debian-lts-announce/2017/11/msg00018.html
https://access.redhat.com/errata/RHSA-2017:3372
https://access.redhat.com/errata/RHSA-2017:3247