This indicates an attack attempt against a buffer-overflow vulnerability in Mozilla Firefox.
The vulnerability is caused by an error when the vulnerable software handles a crafted UTF-8 long URL. It allows a remote attacker to execute arbitrary code via sending a crafted web page.
Mozilla Firefox before 22.214.171.124
SeaMonkey before 1.1.12.
System Compromise: Remote attackers can gain control of vulnerable systems.
Upgrade to the following or later versions: