Earlier quoted context omitted.
Agreed that Mozilla's original mission has been accomplished in spades, but I wouldn't count Mozilla out of the browser race just yet. Servo and Webrender[0] will completely shake up the browser landscape, and will allow web apps to match (maybe even surpass?) native mobile apps in terms of rendering performance. Unless Chrome, IE, and Safari can develop an answer to Servo and Webrender by the time those technologies…
Servo won't be ready for quite some time. (I'm thinking maybe 5 years) Project Quantum might be out sooner but it won't be that big of a leap ahead of other browsers.
So it's not entirely unreasonable to suggest that Mozilla's next-gen engine efforts could be first-to-market, and that everyone else might have to play catch up.