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Be sure to back up your Firefox profile before running Fred's little batch file. It removes many more files than most users would want, including all your bookmarks, passwords, site-specific settings, toolbar configuration, application settings, the security module, and much more. I don't recommend using it at all.
Edit: Bookmarks are not removed.
I echo that sentiment the little program is a disaster (not far short of a piece of malware), removing much, much more, even hacking through installed KB stuff and many more areas totally unrelated to firefox. It left my system unbootable, with an error about hal.dll (I think that is correct) an essential file.
I could have stopped it when I saw it hacking through my system like wildfire, but I left it to see the end result, safe in the knowledge that I would be able to restore my weekly drive image back-up.
So after the un-bootable error, I just popped in my bootable CD to run the Drive Image software and under 30 minutes later I was virtually back to square one having also restored my daily backup for emails, docs, etc. I shudder to think how someone without a robust back-up and recovery strategy would have coped.