About 20% of the time Firefox is run in sandbox, only 3 plug-ins appear; Adobe Acrobat, Java Deployment Toolkit, and Mozilla plug-in. 5 are missing; Google Update, Java Platform, Shockwave flash, Silverlight, and Win. Live PhotoGallery. Wondering if any one else noticed this lately, or if its already known/being worked on.
Is this MS Web Sandbox what the Avast! sandbox is/or is modeled after then? Relevant or not, still good info, so thanks for the link 
I guess the best way to answer your question is that there are a lot of “sandboxes” out there and they all act as a sort of protection to run programs in a more controlled environment. Wikipedia can give you a good description. Avast Pro does have a sandbox.