my mate in our office just got $2,000 from Google adsense as he referred something like 25 people who all made $150 i know he's referred over 400 people now from his ebooks and his blogs and stuff.
I just put the firefox one on three of my proxies because only 10% of visitors are using firefox. Im not loosing any money from putting the buttons on my sites in fact they make my proxies look a little less ad filled so its just icing on the cake.
Firefox Referrals work pretty good for me, although the contextual adsense works better. Adsense not working at all.