This indicates an attempt to exploit one of several vulnerabilities in Mozilla based browsers.
Mozilla based browsers allow remote attackers to bypass the "same origin" policy, steal cookies, and conduct other attacks by writing a URI with a null byte to the hostname DOM property. The vulnerability is due to interactions with the DNS resolver code.
Firefox before 18.104.22.168 and 2.x before 22.214.171.124
SeaMonkey before 1.0.8
Denial of service.
The vendor has released updated versions of the affected application to address this issue.