Search firefox in Google and you can see some people promoting Google's firefox program. I have also seen some people promoting it on the content network through Googlepages.
Well, I've never heard anyone make a fortune with referrals. So, perhaps they do show them, but that's not the reason of their success... The reason, as always, would be original content, which is the key to more visitors, which is the key to more clicks, on either referrals or ads.
Does FF work on all type of sites? I know that adsense referrals are more likely to work on sites that are technology, webmaster oriented.
What do you consider a "big Earner" ? I do very well with FF referrals (about 20%-40% of total Adsense earnings). But your site/page has somehow be related to the subject or type of people that could want/use FF or Adsense.. If your site is about Potatos or something like that, you wont have much luck pushing FF or Adsense.. Check out the new Adsense Case Study, which talks about how a site can do so well with FF referrals.. I know the guy that owns that site Also check THIS POST AT ADSENSE SUPPORT for tips on recommending your referrals for better conversions.