I was under the impression that no one "sold" any open source software, per se (except paying for delivery of the media and some pretty packaging, or things like that). What you pay for is support from a branded company for a branded version of the open source code. I would go pay a company like the one that makes Firefox if my company decides to make it our standard browser (currently 30,000 seats worldwide). We already do this with Red Hat Linux. Installing and using the software is free. We pay them for support. That's the business model.
I ain't paying the folks at GNU squat and, even if I did, they wouldn't be providing me the kind of support I require. Red Hat does a very nice job for us on the servers we use it on.
You can't, AFAIK. Firefox is run by the non-profit 501(c)3 Mozilla foundation. You can make a nice tax-deductible donation though, which will improve features, code quality and support.