This tidbit is actually rather exciting. Firefox uses a rendering engine that reads XML to determin how its user interface is laid out. And now that they’re going to release it as a runtime environment, it should be possible to create desktop clients like one develops for the web. So instead of an API like Swing, or the like, you have a declarative language for rendering layout for desktop applications.
In addition, it should also be possible to transmit user interfaces from application to application and platform to platform more easily with such a declarative language for layout. I imagine being able to drag and drop interfaces from your desktop to your mobile device as a metaphor for “taking things with you.”
I’m sure other people that have been paying attention in this field saw this coming for a while. Adobe’s AIR (Adobe Integrate Runtime) is aiming specifically in this field. We’ll see how this plays out, but likely, developers will choose one or the other for a specific strength, and each will have their own niche. You can try out Firefox 3’s current alpha. It’s been much more stable in the last couple of weeks.