Rated 3 out of 5 stars
Prism is no longer developed by Mozilla Labs. There are now two options:
The Prism code was picked up by Matthew Gertner who started a company to continue its development under the name WebRunner: http://www.salsitasoft.com/blog/2011/02/09/prism-is-now-webrunner/ - a Firefox 4 installer is available.
Mozilla Labs itself started a new project called Chromeless http://mozillalabs.com/blog/2011/02/prism-is-now-chromeless/ . It can be found at http://mozillalabs.com/chromeless/