In certain circumstances a networking event listener can be prematurely released. This appears to result in a null dereference in practice. This vulnerability affects Firefox < 52 and Thunderbird < 52.
https://www.mozilla.org/security/advisories/mfsa2017-09/
https://www.mozilla.org/security/advisories/mfsa2017-05/
https://bugzilla.mozilla.org/show_bug.cgi?id=1328121