If Google stopped supporting Firefox today, Bing would still pay to be the default engine. If bing does not pay, Yandex would do.
My point here is Firefox still has 2.71% market share, a lot of search engines operators would pay Firefox good money to be their default engine.
Their revenue stream is guaranteed as long as they have good amount of users.
But they actively choose to ruin it.
There was a point where Yahoo paid to be the default in America, Google came back at the next opportunity and doubled (IIRC) their offer.