Firefox is renowned for using masses of memory, however, firefox 3 apparently uses less memory than IE (not that ive noticed, but apparently stats show it)
If you havent gof FF3 yet anyway, you probs should get it, it's decent.
I would say that though Firefox does seem to use more memory, Some of IE memory usage is hidden as IE is the desktop (yeah, right) so some of that system memory is actually IE.