Google's main focus is in extracting rent from their dominance, not in making the browser faster, lighter or whatever.
As for plugin support, that's the challenge no? Make it so the contract for third party plugins can be maintained without breaking them every 6 months as the browser improves.
Firefox has excellent developers. The fact that it still has some relevance despite many years of mismanagement is testament to that. I bet if the company behind the browser was laser focused in making it as good as possible, with no compromise, they could challenge Chrome dominant position.