This indicates an attack attempt to exploit a Type Confusion 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 Thunderbird before 60.7
Mozilla Firefox ESR before 60.7
Mozilla Firefox before 67
System Compromise: Remote attackers can gain control of vulnerable systems.
Apply the most recent upgrade or patch from the vendor.