Interesting thing about the memory usage. With the exact same number of tabs (and same corresponding sites), IE appears to use a separate instance for each tab (although it's only showing 5 and I have 6 tabs open?) whereas FF uses only one. Note this does not include "plug-ins" such as IE's activeX controls or FF's eponymous containers.
for IE9 and 168,568
for FF 4.0b7pre. Big difference. Remember, this is with the exact same sites open.