Vitalsecurity.org has an interesting report of a potential spyware/adware infestation that directly attacks Firefox users, using the Sun Java Virtual Machine as its installation engine. (In the comments, a security expert from the Mozilla Foundation notes that this exploit could attack Opera users as well.)
I’ll look at this in more detail when I get back in the office.
Ed Bott 
Well, I think it’s true, after installing the JRE for using some applet on Firefox ny antivirous detected some abnormal movement in a jar. Dunno if those are related but it was the only way of intrusion