Weak proxy objects have weak references on multiple threads when they should only have them on one, resulting in incorrect memory usage and corruption, which leads to potentially exploitable crashes. Note: This issue only affects Firefox for Android. Other operating systems are not affected. This vulnerability affects Firefox < 51.
https://www.mozilla.org/security/advisories/mfsa2017-01/
https://bugzilla.mozilla.org/show_bug.cgi?id=1293709