Memory safety bugs were reported in Firefox 53 and Firefox ESR 52.1. 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 < 54, Firefox ESR < 52.2, and Thunderbird < 52.2.
https://www.mozilla.org/security/advisories/mfsa2017-17/
https://www.mozilla.org/security/advisories/mfsa2017-16/
https://www.mozilla.org/security/advisories/mfsa2017-15/
https://www.debian.org/security/2017/dsa-3918
https://www.debian.org/security/2017/dsa-3881
https://access.redhat.com/errata/RHSA-2017:1561
https://access.redhat.com/errata/RHSA-2017:1440