They've started on it, it's called Servo. But webstandards are evolving faster than modern browsers are, so building a browser from the ground up would require

They've started on it, it's called Servo. But webstandards are evolving faster than modern browsers are, so building a browser from the ground up would require quite a bit more money, and worse know-how - you can't just buy that in unlimited amounts, than they have for Firefox right now, and they can't exactly stop developing Firefox in the meantime either. Also, their three big competitors have most of their browser market share thanks to building an operating system underneath it. No matter how slim Mozilla's chances at success were, it would've been foolish to not try to get into the operating system market. And they built it based on web technologies, because that's where they have know-how.