This indicates an attack attempt against a memory corruption vulnerability in Mozilla Firefox.
The vulnerability is caused by an error when the vulnerable software handles a specially crafted webpage. It allows a remote attacker to execute arbitrary code.
Mozilla Firefox 3.0.10 and prior
System Compromise: Remote attackers can gain control of vulnerable systems.
Denial of Service: Remote attackers can crash vulnerable systems.
Upgrade to Mozilla Firefox version 3.0.11: