The Prism project (formerly WebRunner has recently been announced by Mozilla Labs.
In this announcement, a sentence has been ruffling a few feathers at Adobe and Microsoft (emphasis mine):
Unlike Adobe AIR and Microsoft Silverlight, we’re not building a proprietary platform to replace the web. We think the web is a powerful and open platform for this sort of innovation, so our goal is to identify and facilitate the development of enhancements that bring the advantages of desktop apps to the web platform.
Adobe, Microsoft and Mozilla are platform vendors. Adobe pushes Flash, Microsoft pushes .Net and we, at Mozilla, push the Open Web. And recently, Adobe has announced AIR (Adobe Integrated Runtime), Microsoft has announced Silverlight and some people at Mozilla are working on Prism. Why these three organizations have been doing this recently? To extend their application development platforms (and leverage their numerous developers) to places where it is either weak or non-existent before:
- Adobe pushed Flash from the browser to the desktop with AIR
- Microsoft pushed the .Net stuff from Windows to the browser
- Mozilla pushed the Open Web technologies from the browser to the desktop
So, in a way, Adobe and Mozilla moves are the same, except that Adobe promotes their proprietary technologies, while Mozilla promotes the Open Web and make its applications easier to use and more integrated with the desktop. Proprietary technologies give more control and power to the software vendors, while Open technologies (like the Open Web) give more freedom to the users. By promoting proprietary technologies, Adobe and Microsoft are trying to squeeze more value from the developers and pass it over to their shareholders. By doing the same around open technologies, Mozilla is creating value for the public good.
P.S.Oh, I have found in this comment to the Microsoft post something very similar to my thinking:
MS says "Write a web application using Silverlight and you can get desktop integration". Adobe says "Write a web application using AIR and you can get desktop integration". Mozilla says "write a website *without* doing anything special, and *we* will take care of desktop integration".
 You can find more details about Prism on Mark Finkle's post and on the Prism forum.