Why would anyone bid for being the search engine in a browser with <4% market share and falling? Everybody will want to be on iPhone/Safari, Android/Chrome, and Windows/Chrome or Edge.
If I set my search engine to DDG and Firefox has a deal with Google, then my browser is going to change my search engine to Google? Of course not. When you install Firefox it defaults to Google. In the past when Firefox had a deal with Yahoo it defaulted to Yahoo.
Are you suggesting that is incorrect because all of that happened?
And at least 50% of this 4% (myself included) use ad blockers. Well, too bad Mozilla put itself in this situation. If it doesn't kill it, it will make it stronger.