Hundreds of developers?

It's a large code base, but it doesn't require hundreds of developers working concurrently to keep it up to date. Web standards move fast, but not that fast.

Take a look at Ladybird. There's a browser being built from scratch with a small team (less than 10?)

If Mozilla fired 90% of it's employees and kept the 10% to actually work on Firefox, it could be a great browser.