Mozilla developers and community members reported memory safety bugs present in Firefox 63 and Firefox ESR 60.3. 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 Thunderbird < 60.4, Firefox ESR < 60.4, and Firefox < 64.
https://www.mozilla.org/security/advisories/mfsa2018-31/
https://www.mozilla.org/security/advisories/mfsa2018-30/
https://www.mozilla.org/security/advisories/mfsa2018-29/
https://www.debian.org/security/2019/dsa-4362
https://www.debian.org/security/2018/dsa-4354
https://usn.ubuntu.com/3868-1/
https://usn.ubuntu.com/3844-1/
https://security.gentoo.org/glsa/201903-04
https://lists.debian.org/debian-lts-announce/2018/12/msg00002.html
https://access.redhat.com/errata/RHSA-2019:0160
https://access.redhat.com/errata/RHSA-2019:0159
https://access.redhat.com/errata/RHSA-2018:3833