This indicates an attack attempt to exploit a Integer Underflow Vulnerability in Mozilla Foundation Firefox ESR.
The vulnerability is due to an error in the vulnerable application when handling a maliciously crafted response. A remote attacker may be able to exploit this by tricking a user to visit a malicious web page and execute arbitrary code within the context of the application.
Mozilla Foundation Firefox prior to 58.0
Mozilla Foundation Firefox ESR prior to 52.6
System Compromise: Remote attackers can gain control of vulnerable systems.
Apply the most recent upgrade or patch from the vendor: