Crap Cleaner (CCleaner) is ur best bet. Light wieght, small yet effective. Cleans Firefox's catche and cookies (you can disable it or exclude certain cookies).
MRU Blaster is another good program. Hasnt been updated for Firefox but great program to clean MRU (hence the program's name).
But Window Washer is a very great program. Probably one of the most advance cleaners out there. I suppose run this tool along with CCleaner and u'll be set.
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