I was under the impression that Mozilla also got a sizable chunk of revenue from Amazon. Every time I search Amazon, I do it via the browser search box, and there's a referral code appended. Even if Mozilla doesn't have a special agreement with Amazon (this would not surprise me), they would no doubt qualify for Amazon's highest volume-based advertising rate, which is 8.5% on all sales (excluding electronics and gift cards, which have a rate of 4% regardless of your referral-volume tier).

I presume that the same is also true of eBay in some fashion.