How does Firefox make money?

The Mozilla Foundation makes most of its money from Google for referrals from the default Firefox home page and built-in search box.

Ding Ding Ding you are correct sir!

Mozilla gets donations from it's users, development grants, and advertising revenue from companies like Google.

as does opera

…but Opera doesn’t receive an annual amount that makes up most of its income from Google just because they use it as default search engine in the search box. In fact, this search box was also considered as a strong point for Firefox so it’s a double-win for Mozilla :)
Now that I mention it: Firefox doesn’t make money, the Mozilla Foundation/Corporation (don’t know which one) does.