Chromeless vs Prism/WebRunner
Prism (an ex-Mozilla Labs project, now discontinued but kept going independently of Mozilla under the WebRunner name) was/is a way to make Web pages superficially appear to be native applications. Firefox users could choose to turn a Web page into a "Prism application" and have an icon added to their desktop to launch it without the Firefox user interface surrounding the page. However, Prism "applications", being Web pages, were subject to all the normal restrictions that browsers place on a Web page. For example, Prism applications could not read/write files directly from/to the computer's file system. One of the ideas behind Chromeless is to remove these restrictions.