This indicates an attempt to exploit a Domain Name Spoofing vulnerability in Mozilla based browsers.
The vulnerability is caused by an error when the software handles a "hostname DOM property" with a null byte. It may allow remote attackers to bypass the "same origin" policy.
Firefox before 18.104.22.168
Firefox 2.x before 22.214.171.124
SeaMonkey before 1.0.8
Upgrade to the latest version,available from the web site: