Summary
Mozilla has published an out-of-band patch for Mozilla Firefox and Firefox Extended Support Release (ESR). It fixes a type confusion vulnerability in Mozilla’s Javascript compiler, IonMonkey. This vulnerability is identified as CVE-2019-17026. [1]
Mozilla’s advisory states they are “aware of targeted attacks in the wild abusing this flaw.” Based on this note in the advisory, it appears the vulnerability was exploited in the wild as a zero-day. [2]