Google is the largest financial supporters in Firefox, they buy Firefox search bar. If they wanted to destroy Firefox, they’d already do it by simply cutting that revenue stream.
They can't do it like that directly, but they have been undercutting Firefox for years. For a long time when you searched Google from Firefox it would tell you to download Chrome.
Or some other search engine would become their revenue stream, like when Yahoo was the default search. Without Google bidding their revenue would likely drop, but it wouldn't disappear.
True. But revenue would be significantly smaller (other engines cannot afford pay high per click, and lack of competition drives the price per click down), and revenue is already barely enough to cover Mozilla needs (see recent staff cuts).