This indicates a possible attempt to exploit a buffer overflow vulnerability in Mozilla Firefox.
The vulnerability is caused by the inability of the vulnerable software to handle an allocation failure. A remote attacker could exploit this vulnerability by sending an HTTP index response with a crafted 200 header. It will lead to a crash of the targeted Firefox.
Mozilla Firefox 3.0.4 and prior
SeaMonkey 1.1.13 and prior
Denial of Service.
Apply a patch, available from the web site