You can't just maintain a browser, the web is ever evolving.
That'd be a good way to get left behind.
Even now there are technologies that chromium supports and ff doesn't, e.g. the new-ish webusb api. (Actually checking now it is supported as experimental, but my point stands)
I think the point is that the world is constantly moving. If something isn't maintained properly the technical dept builds up to the point where it can't be salvaged. (See Xorg)
That's a valid question. Unfortunately, it's difficult to quantify.
The state of browsers in general has been a moving target since NCSA Mosiac; about around 1993 or so. So the last three decades has been a ceaseless grind of new features, security enhancements, performance enhancements, and so on. And this feature set is absolutely monstrous in scale, as it includes backwards compatibility to most of those features (if not all of) back to that beginning over 30 years ago. So, work on any browser is by definition perennial, and it only ever gets more complex.
I would also argue that the only other software projects that compare to a web browser in terms of sheer scale, compatibility, and longevity, are things like the Linux Kernel or maybe the entire Microsoft Office suite. IMO, software in this class is a lot of work to keep going, no matter how you slice it.
A browser is one of the most complex pieces of software you will find. There's a reason why only 2.5 browsers exist (I'm counting chromium and safari as 1.5 because they are not the same but they are both WebKit). Maintaining a browser is difficult and making a new one is even more difficult.
Take Microsoft, one of/the most valuable company in the world. They had a browser (internet explorer) that has been state of the art, then they couldn't maintain it anymore and it became a joke. They made a new one instead (old edge) with all the intention of making it a real player. Fucking Microsoft couldn't do it and had to give up. They replaced it with a reskin of chrome (new edge).
Apple and Google manage to maintain chrome and safari both thanks to their position of monopoly, and because their position of monopoly depends on it. Firefox exist(ed) as a tax sponge for Google, but it's definitely behind chrome in technology, but if it was a new browser, and not one order than safari, they would never be able to make it.
It's around 30 million lines of code. You need actual human beings who have enough knowledge of this code to make decisions.
When I'm on a project with 30000 lines of code as a reasonably experienced dev, I consider that rather challenging to know most details of. This is obviously some complete ballpark math, but that would mean they need 1000 devs.
They had around 750 employees in 2020, after they laid off 250 employees. This includes HR, management, IT support and such, so possibly 650 actual devs, of which not all are working on Firefox.