The comments about offline functionality are a little misleading.
Firefox 3 supports running Web applications offline via the HTML5 Offline Apps caching spec.
This is built into Gecko 1.9. Firefox 3.1 will support some additional features of that spec.
The spec is similar to the offline support in Google Gears.
Prism is orthogonal. It's a way to wrap up a Web site in a sort of custom browser app, so you
can have an app that's just a shell around GMail, for example. The Web app could use the HTML5
Offline support in Gecko 1.9, but it doesn't have to. Completely orthogonal.