Google has already given them such a huge amount of money that if Mozilla managed it well and ran a lean organization they could coast decades without further payments. Instead they seem to spend it as fast as they get it.
They've burned up billions with little to show for it (their declining market share is the proof of this) and now wonder how they will survive.
There are ways they could make money and leverage their browser, such as becoming a certificate authority (they're literally outsourcing profit) or making their own search engine (which would probably be a bad idea for other reasons). Nothing else is going to replace $400m/year.
Their real problem is not "making money." It's too much easy money making them go soft, and then wasting it.