Google is giving them so much money, for such a long time, that it is hard to replace. Golden handcuffs. Donations and $10/mo "Firefox subscriptions" aren't going to make up for it.

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.