The bad guys are using the FireFox extensions as a means of piggybacking FireFox to steal sensitve user data.
Once FormSpy is executed, it installs itself as a component of the Firefox Web browser.
The FormSpy spyware then gleans sensitive information, such as credit card and bank account numbers, from the user’s browser and forwards it to a malicious Web site. But this Trojan is capable of other tricks, as well, McAfee noted.
Read more here.