Servo is just the browser engine, an alternative to the component known as Gecko in modern Firefox. Mozilla isn't going to drop the Firefox brand even if they do end up replacing Gecko with Servo (which is hardly guaranteed). Though if anything, the move here makes it more plausible that such a switch could happen, because the whole XUL situation is probably the single largest blocker for a Servo-based Firefox. A standardized extensions API would also give Servo a free extensions ecosystem with minimal effort, especially if Mozilla pursues a GUI based on browser.html which could be shared with Servo immediately.