This looks neat - my friend Richard Birkby has put together a Firefox version.
This morning I'd thought I'd try and help out by building the viewstate parser for Firefox. He beat me to it though. However it only works for Asp.Net 2.0.
My parser is a bit brute force compared to Rich's, but it might be useful for .Net 1.1 users to have a firefox sidebar showing the ViewState breakdown. Plus it'll be a nice introduction to writing XUL widgits.