The web nowadays is not only distant html sites, it is also rich internet applications. We can consider desktop applications as rich internet applications because more and more of this desktop applications are based on web services, are linked intimately to the web, to the internet. And then more and more technologies allow to merge the web and the desktop. So in the future, Firefox will represent only a part of the purpose of the web. So I think the development of a platform for Rich Desktop Application follow the goal of Mozilla Fundation : maintaining the choice and the innovation on the Internet.

And because XulRunner is a great platform for this kind of application, and because the new web shouldn't fall only into the hand of Microsoft (through Silverlight,Xaml...) and Adobe (through Apollo, Flex...), Mozilla should invest more on XulRunner. So, as Daniel said:

Mozilla, please, turn XULRunner into a Mozilla product !