This indicates an attack attempt against a security bypass vulnerability in Mozilla Firefox.
The vulnerability is caused by an error when the vulnerable software handles a "for" attribute in a label. It allows a remote attacker to bypass the software's "focus prevention" via sending a crafted web page.
Mozilla Firefox 220.127.116.11, 18.104.22.168 and other versions before 22.214.171.124
SeaMonkey before 1.1.5
Upgrade to the latest version.