A use-after-free vulnerability can occur when working with XMLHttpRequest (XHR) in an event loop, causing the XHR main thread to be called after it has been freed. This results in a potentially exploitable crash. This vulnerability affects Thunderbird < 60.7, Firefox < 67, and Firefox ESR < 60.7.
https://bugzilla.mozilla.org/show_bug.cgi?id=1542465
https://www.mozilla.org/security/advisories/mfsa2019-13/