I was wondering about the same thing. I've got a good, working IE "fix" for the fixed position issue, and would like very much to have the menu be fixed.
I've tried lots of stuff with css, and got the root menu to stay put, but the submenus keep moving. I would imagine that the submenus are placed explicitly by the menu system each time there is a mouseover, so changing any of the css is futile.
Anyway, I've placed the menu in a div that is fixed in both IE and firefox, and the menu part stays fixed, but the submenus keep moving with the rest of the text.
The test site can be found at
http://nutcates.com/moe/
anyway, just my 2 cents