There is a large discussion already, it depends on what kind of referrals they come up with, this is only the beginning but one could think about advertisers that sell products like i-pods or cd's... the firefox one is nice for certain sites but not every site will make something out of it. When you have a site about browser compatibility the download link could work but on a site about for example dogs? Very unlikely...
I believe that the Firefox and Google team effort will be a good push to get IE out of the way. While many people still be using IE, FF will take a larger portion of the market in the coming months.