Memory safety bugs were reported in Firefox 55 and Firefox ESR 52.3. 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 < 56, Firefox ESR < 52.4, and Thunderbird < 52.4.
https://www.mozilla.org/security/advisories/mfsa2017-23/
https://www.mozilla.org/security/advisories/mfsa2017-22/
https://www.mozilla.org/security/advisories/mfsa2017-21/
https://www.debian.org/security/2017/dsa-4014
https://www.debian.org/security/2017/dsa-3987
https://usn.ubuntu.com/3688-1/
https://security.gentoo.org/glsa/201803-14
https://lists.debian.org/debian-lts-announce/2017/11/msg00000.html
https://access.redhat.com/errata/RHSA-2017:2885
https://access.redhat.com/errata/RHSA-2017:2831