[rglullis]

According to a contract, which can be renegotiated or simply not renewed.

Google does not pay Apple for "access to the user base". Google pays Apple so that Apple does not try to get into the search engine space and compete with Google.

They also pay Mozilla as a way to keep controlled opposition and a (relatively) inexpensive way of avoid threats of lawsuits like the one that MS had in the past. But they don't need to. If Mozilla for any reason decided to stop conforming to this game and went on to actually fight against Google, Google would drop them in a heartbeat. And who would Mozilla go for, Bing? DDG?