This indicates an attack attempt against a remote code-execution vulnerability in Mozilla Firefox software.
The vulnerability is a result of the application's failure to handle malicious calls to the XUL tree method _moveToEdgeShift(). As a result, a remote attacker can send a crafted HTML file to execute code on a vulnerable system.
Mozilla Firefox older than 3.0.8
System Compromise: Remote attackers can gain control of vulnerable systems.
Update your software to at least version 3.0.8