This indicates an attack attempt to exploit a Same Origin Policy Bypass vulnerability in Firefox.
The vulnerability is due to an error when the vulnerable software attempts to handle a maliciously crafted webpage. An attacker can exploit this by tricking a user into visiting a malicious webpage and execute arbitrary script code within the context of the application.
Mozilla Firefox before 42.0 and Firefox ESR 38.x before 38.4
System Compromise: Remote attackers can execute arbitrary script code within the context of the target user's browser.
Apply the most recent upgrade or patch from the vendor.