I don’t really agree with you. Yes, if Firefox disappeared, it would be terrible because there would be a duopoly. But don’t forget that there are alternatives to Google Chrome/Microsoft Edge. Brave, Opera, Vivaldi… These will never cease to exist as long as chromium exists. Brave will always block ads just as well, because its ad blocker is directly integrated into the browser. So despite the V4, V5 and V1000 manifests, ads and trackers will continue to be blocked.
Paradoxically, the demise of Firefox could lead to the rise of Vivaldi and Brave, which are far more respectful of privacy. Not to mention ungoogled chromium
So even if chromium has the monopoly, it won’t prevent the birth and growth of alternative browsers.