Their relationship goes as deep as Firefox bringing Google as the default search engine. I presume Google pays some non-negligible amount of money for that, so

Their relationship goes as deep as Firefox bringing Google as the default search engine. I presume Google pays some non-negligible amount of money for that, so Mozilla execs are tied to Google. Having one big source of income implies pivoting your whole strategy on virtually anything at their command because without them, you can kiss goodbye the "between $20 million and $100 million" marketing budget.