I asked myself this very same question. I think the answer is: you don't, not directly. People simply don't care enough about their browser to pay anything.
I think we need to take a step around this problem. Here are two suggestions.
Maybe Mozilla can sell a VPN service or some other kind of product that is external to Firefox. They then use the revenue to fund Firefox development.
Mozilla can also launch an alternative version of Firefox that they label 'privacy-optimized'. That version will have all tracking disabled by default. People who really care about privacy will use that version, and it will solve their biggest complaint of 'having to turn off yet another opt-in feature'. Meanwhile, the vast majority of users don't care about tracking, so Mozilla can enable tracking by default in the mainstream version.
