Google pays you a dollar for each Firefox download you create from your AdSense account, it is called AdSense FireFox referrals. But Google doesn't like it when you tell people to click on your AdSense ads or if you tell people to take a desired action.
That is the topic of a recent WebmasterWorld thread.
The thread creator asks, can he place on his site, "This site is best viewed in FireFox" and then directly under that, have a Firefox referral button?
A friendly member replies that the AdSenseAdvisor replied to this question back in January stating;
To clarify, you cannot use language like, "click here" or "click on this button to download Firefox", or display a giant arrow drawing undue attention to your referral button. However, in the content of your site you could provide an endorsement for the product you are choosing to refer. For example, you might say, "I recommend the Firefox browser."
Forum discussion at WebmasterWorld.