A use-after-free vulnerability occurs during certain text input selection resulting in a potentially exploitable crash. This vulnerability affects Thunderbird < 52.1, Firefox ESR < 45.9, Firefox ESR < 52.1, and Firefox < 53.
bugzilla.redhat.com/show_bug.cgi?id=1443332
www.mozilla.org/en-US/security/advisories/mfsa2017-11/#CVE-2017-5432