Google doesn’t fund Mozilla anymore than I fund my landlord or my mobile carrier. Google pays Mozilla (and many others) for digital real estate (the privilege of occupying the default search engine slot). There is a lot of confusion and misinformation about this relationship, many people mistakenly characterize it as patron/client but it is more akin to landlord/tenant or vendor/customer (where Google is the tenant/customer). I’m not saying that is how you characterized it, I just wanted to clarify a common source of confusion.
Mozilla has a thing of value (the default search provider slot, for their browser), and they lease that to the highest bidder (presumably), which is currently Google, in the past was Yahoo, in the future will be ____?
But the point I’m making is their is no benevolence, charity, patronage or anything else involved in this arrangement, Google can’t just simply “pull funding”, Mozilla has something Google and others want, and Google is willing to pay a lot of money for that thing. Google gets as much out of the relationship as Mozilla does, and if/when that relationship ends, there will be other interested parties.
That said, more diverse and distributed revenue streams should be (and is) a goal for Mozilla. Unfortunately a vocal subset of users seem to want to have their cake and eat it too, they complain about dependence on the search deal with Google, but then complain even louder whenever Firefox does attempt to introduce some alternative revenue stream that would reduce reliance on revenue from the search deal.