CCleaner the saved form information in firefox. it only cleans it in IE. can this be extended to firefox? I think that'd be a cool update.
Quick Guess that you're using ff3 there are a ton of posts here about ff3 not cleaning, I'm sure the developers are hard at work fixing this, until that time why not just set your fox to kill that information when you close it, or use the program "iehistory eraser" which AFAIK works with FF 3 (Don't know for sure because my portable FF is set to delete private/history data at close)