It's literally the only reasonable [0] non-chromium browser left. Everything else builds upon Google's Chromium, including Brave and Edge. If Firefox dies, the Chromium implementation, which is controlled by Google, becomes the web standard, even more than it already is, with all the drawbacks. Of course, you could fork it, but given that all except one competitor have left the scene and the insane amount required to maintain a browser engine, this is not going to happen. We're lucky Mozilla has already sunk that cost.
[0] Yes, there's Lynx and some hobby browsers and yes, there are people who use them as dailys. But you can not reasonably expect anyone to work with them or get a non-technical person to live with those.
Forks of Chromium are not controlled by Google. Google gave up the legal right to restrict what other parties can do with Chromium when they released it under a liberal open-source license. Google has a little influence over Chromium in that the people that designed and implemented Chromium mostly still work for Google.
Google has more control over Mozilla than they do over organizations that have forked Chromium because about 90% of Mozilla's revenue comes from Google.
Technically there is WebKit which powers safari and gnome web. The good thing is that iOS app store basically forces you to use WebKit for websites so it won't die. Bad news is that Apple does not really like the new web standards and in the name of privacy simply doesn't implement standards that can give web apps an edge over native iOS apps. And Gnome Web currently has licencing issues with webrtc and media support is poor(gstreamer). Also Chromium is a WebKit fork pre 2013 so it shares several quirks with Safari(prob some of the old standards) which is a prob in itself
> If Firefox dies, the Chromium implementation, which is controlled by Google, becomes the web standard
But this just isn’t true - just like Linux (which dominates the web server space) there are multiple parties working on and forking chromium - if google introduced something someone didn’t like they can maintain their own fork and plenty of people are.
Additionally there are other alternatives that enjoy 5x+ the market share of Firefox, most notably, safari/WebKit.
Maintaining a fork becomes progressively harder as upstream adds features or does code cleanup. And with the pace of modern standard development, this happens sooner rather than later. So in practice, all major Blink-based browsers seem to use codebase that closely tracks upstream, if not upstream itself.
[0] Yes, there's Lynx and some hobby browsers and yes, there are people who use them as dailys. But you can not reasonably expect anyone to work with them or get a non-technical person to live with those.