it depend on where the person who has downloaded it comes from. If they come from some poor country you may get only 10 cents
I'm not sure with that. Of course, Firefox (with his 10 cents) is not worthy, but it is different with ads. Income is lower because advertisers put lesser money to the campaigns. Not because of where from the visitor is. My opinion.
I would refer people to Google to download Firefox for free. Are you kidding? It's so much easier to design for Firefox. If I had 100% firefox traffic, that would make my life so much better. it's awesome that I get paid to do it, but I would do it anyway (For IE browsers only)