Firefox has long been considered Spyware hardened and spyware safe, it never really was. Don't get me wrong on this, it's not the fault of Firefox (although it could be a bit better protected against this particular attack). I made a small movie demonstrating this particular Proof of Concept.
Update: A bit of clarification what this fuzz is all about, as you see in the small animation, the Extension installs without any user interaction. That should be quite new, Firefox tries to block silent installs though random profile directory names and various other tricks. The adbar sends any URL you visit to a google syndication server thus monitoring your surf behaviour.
Update : The animation takes some time to load, wait for it.