The
IE-to-Firefox migration guide would be a good start for research.
JS 1.6 -which is part of FireFox 1.5- also introduces
some new features, which obviously IE wouldn't understand.
In addition, "window.external" in IE comes to mind as something that isn't part of FireFox or any standard.
[ February 08, 2006: Message edited by: Ulf Dittmer ]