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There isn't a source, because Mozilla doesn't break it down. They just say that they get almost all their revenue from "search royalty payments", and their two biggest deals are with Google and Baidu. You just have to guess how much comes from Google (90% market share worldwide) vs Baidu (the biggest search engine in China, which is also the largest single internet market but not as well funded as the US market).

https://www.mozilla.org/en-US/foundation/annualreport/2016/

> Mozilla Foundation philanthropic programs and activities are funded by public support from individual donors and foundations ($13.8M), as well as from royalties earned that are paid by the Mozilla Corporation ($8.3M). Total revenue and income support to the Mozilla Foundation in CY 2016 was $23.4M.

> The majority of Mozilla Corporation’s revenue is from royalties earned through Firefox web browser search partnerships and distribution deals around the world. Mozilla Corporation’s revenue and income support for CY 2016 was $506M

https://www.computerworld.com/article/2500712/google-to-pay-...

> Google to pay Mozilla $300M yearly in new search deal, says report

Only goes up to 2011 but https://en.wikipedia.org/wiki/Mozilla_Corporation#Google