This indicates an attack attempt to exploit a Heap Buffer Overflow in Mozilla Firefox.
The vulnerability is due to an error in the HTML5 parser when handling a maliciously crafted HTML file. A remote attacker may be able to exploit this by tricking a user into visiting a malicious web page and execute arbitrary code within the context of the application.
Mozilla Firefox before 47.0 and Firefox ESR 45.x before 45.2
System Compromise: Remote attackers can gain control of vulnerable systems.
Apply the most recent upgrade or patch from the vendor: