In the old days, a few motivated nerds could write a browser. Now all you can realistically do is take a browser engine and build some user interface around it. That what most "alternative browsers" do - tweaking or repackaging.

These days, a browser is like it's own operating system with sandboxing, various Interfaces to periphery devices, hardware acceleration for GPU and all the bells and whistles taken for granted now.

I'd say that imagining it to be on a scale similar to working on the Linux Kernel is more right than wrong.

So we definitely very much want Firefox to survive, or it will be much worse than the Linux/Mac/Windows trilemma. Microsoft Edge is chromium under the hood too. Any many desktop "apps".