From the 'Good Ideas Gone Bad' files:

Back in 2007, Mozilla launched the Prism effort to create web applications that could run on the desktop. Essentially Prism offered users a way to run a browser loaded to a specific web application.

After a couple of years of active development, Prism development stalled in February of 2010 and there have been no updates about the project or its releases -- until now. Prism could run as an add-on for Firefox (but only 3.6.x and not 4), and as such Firefox 4 Beta users have been missing out on what I believe to be - a fantastic Mozilla technology.

I personally have no idea why Mozilla stopped working on Prism or why it didn't get updated to the Firefox 4 era. The first news that I've seen on Prism in nearly a year came this week with the announcement from Mozilla that Prism is now being integrated into Chromeless -- Mozilla's effort to enable users to build thin browsers.

Interesting idea - but a mistake (in the short term) in my view.