In 2011 Mozilla income was 85% derrived from Google, through the primary search engine deal. Around a billion was paid over three years as part of this deal at some point. Appearantly there was bidding by Microsoft for making Bing the default, which pushed up the pricing.
So every time Mozilla speaks out against Google, it is a bit awkward, since they are biting the hand that feeds them. I suppose they could take a deal from Microsoft, Yahoo or even DDG (or Baidu!), but without interest from Google I presume the funding would be lower. Quite an interesting situation. Thank God both Firefox and Chrome are open source. That is at least some small degree of insurance against potential freedom-limiting shenanigans by tech giants.