A use-after-free vulnerability can occur during font face manipulation when a font face is freed while still in use, resulting in a potentially exploitable crash. This vulnerability affects Thunderbird < 52.6, Firefox ESR < 52.6, and Firefox < 58.
https://www.mozilla.org/security/advisories/mfsa2018-04/
https://www.mozilla.org/security/advisories/mfsa2018-03/
https://www.mozilla.org/security/advisories/mfsa2018-02/
https://www.debian.org/security/2018/dsa-4102
https://www.debian.org/security/2018/dsa-4096
https://usn.ubuntu.com/3544-1/
https://lists.debian.org/debian-lts-announce/2018/01/msg00036.html
https://lists.debian.org/debian-lts-announce/2018/01/msg00030.html
https://bugzilla.mozilla.org/show_bug.cgi?id=1425000
https://access.redhat.com/errata/RHSA-2018:0262
https://access.redhat.com/errata/RHSA-2018:0122