The only reason Mozilla matters in the eyes of Google is because it gives the impression there's competition in the browser market.

But Firefox's users are the kind who choose the browser, not use whatever is there. And that choice is driven in part by having solid ad-blockers. People stick with Firefox despite the issues for the ad-blocker. Take that away and Firefox's userbase dwindles to even lower numbers to the point where nobody can pretend they are "competition". That's when they lose any value for Google.

Without the best-of-the-best ad-blocking I will drop Firefox like a rock and move to the next best thing, which will have to be a Chromium based browser. I'll even have a better overall experience on the web when it comes to the engine itself, to give me consolation for not having the best ad-blocker.