IIRC they got at $300m/year for having Google as the search provider in the top right box.
Not only that, but they added a clause to their contract with Yahoo that let them get out of it while still forcing Yahoo to pay if Yahoo was purchased by another company. When Verizon bought Yahoo, Mozilla switched their default search engine back to Google. I believe they're receiving double pay from both search companies for the next year or two.
I remember when the Vz-Yahoo deal was going down, and realizing how all this would play out. That they got that deal, and now are reaping double pay like that...incredible.
Every year(?) all the big search engine providers make an $$$ offer to Mozilla to become the default search engine in Firefox for anyone who newly installs it.