I actually work for a startup who's flagship application is written on top of xulrunner. I'd say some of the complaints here are valid, it is at times slow. However, we do have the advantage of the future integration of tracemonkey!
Also remember that xulrunner apps become instantly cross-platform because of their nature - think of all these other apps that you're comparing to and ensure you take that into consideration. Is Windows MP cross-platform with all the same features?
Also remember that xulrunner apps become instantly cross-platform because of their nature - think of all these other apps that you're comparing to and ensure you take that into consideration. Is Windows MP cross-platform with all the same features?