A quick look at the Wikipedia article for the corporation says that Google pays the corporation:
"In 2006, the Mozilla Corporation generated $66.8 million in revenue and $19.8 million in expenses, with 85% of that revenue coming from Google for "assigning [Google] as the browser's default search engine, and for click-throughs on ads placed on the ensuing search results pages."
Sources linked there like [0], albeit quite old now, mention only the corporation and not the foundation.
"In 2006, the Mozilla Corporation generated $66.8 million in revenue and $19.8 million in expenses, with 85% of that revenue coming from Google for "assigning [Google] as the browser's default search engine, and for click-throughs on ads placed on the ensuing search results pages."
Sources linked there like [0], albeit quite old now, mention only the corporation and not the foundation.
[0] https://www.theverge.com/2011/12/5/2612918/future-of-firefox...