I doubt Firefox will ever take up as much memory as IE. You forget that most of IE doesn't appear in the list of system processes. Lots of it is embedded in Windows, where it's hard to count. The tiny iexplore.exe "program" that starts IE is just a tugboat that's pulling a huge battleship.
Firefox does have a memory cache, though, and it can be big. That's for dial-up users who want to squeeze out every drop of performance. If you're on a corporate intranet you can turn that off. Type in this URL: "about:config" and search for "memory". Right click on a likely item, change it, and restart.
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