If it helps, you've definitely got some js running in your gecko implementation that is setting the height style to an explicit px size - as part of the page load/editor initialize. Using firefox/firebug you can see the editors height style get set to a fixed px after a page load. Resize the browser and refresh the page - see how the editor gets a new "hardcoded" px size. This initial sizing code is not being run on window resize events. 

 

Also your latest page snippet is not correctly filling the IE7 browser - I'm getting vertical scroll bars (about 20% of the editor is below the window).