Since I don't/can't run IE9 I can't give a comparison, but at the moment:
FF 4.0.1, with 9 tabs open, I'm seeing 144MB usage. (2 being SF, 8 being Slashdot, and 1 on the Uni Website with a PDF open via the Acrobat plugin). The highest it's been is around 900MB, and that's after watching YouTube...