This indicates an attack attempt to exploit a memory corruption vulnerability in Mozilla Firefox.
The vulnerability is a result of the application's failure to properly handle multiple "rdf" file loading in an "xul" tree element. Attackers could exploit it to execute arbitrary code.
Mozilla Firefox prior to 3.5
Mozilla Firefox prior to 3.0.12
System Compromise: Remote attackers can gain control of vulnerable systems.
Upgrade to the following versions or higher.
Mozilla Firefox 3.5
Mozilla Firefox 3.0.12