Freshstart...not quite Rated 2 out of 5 stars
There is an extension for Chrome called FreshStart. I have yet to find anything that is similar to it for Firefox. This is probably on the way towards something similar. However, Freshstart DESTROYS this add-on in terms of usability and flexibility. I would recommend using that extensions as a baseline for functionality.
BTW, I can't create a new snapshot in Firefox 11 beta. I can only save over the existing one.