This indicates an attack attempt to exploit a Memory Corruption vulnerability in Mozilla Firefox.
The vulnerability is due to an error when the vulnerable software handles a maliciously crafted web page. An attacker can exploit this by tricking an unsuspecting user into visiting a malicious web page and execute arbitrary code within the context of the application.
Mozilla Firefox ESR prior to 45.7
Mozilla Firefox prior to 51
Mozilla Thunderbird prior to 45.7
System Compromise: Remote attackers can gain control of vulnerable systems.
Apply the most recent upgrade or patch from the vendor.