Many haven't seen our research paper on this matter we wrote back in March. Here is a link to our whitepaper on Firefox.
In summary, over the past few months security experts have speculated that we could see new spyware and adware crafted for Mozilla Firefox
Until now most of this speculation has centered on the potential for security holes in Firefox that could be exploited by spyware and adware creators. But such speculation is arguably misguided. In fact, we already have seen instances of spyware and adware that can be installed through Mozilla Firefox.
These cases have only received little attention, however, largely because researchers have been too focused on security exploits instead of the more run-of-the-mill methods through which spyware and adware are typically delivered to users' desktops: namely, social engineering techniques that spring unwanted software on confused users who aren't careful enough about what they click.