The problem with most "similators" now days is that they're based on the browser's actual executable. For example if you have IE7 or IE8, you've just lost the ability to render IE6, especially since Microsoft never released a full work-up of all the expected behaviors and codes and such.
It is possible for Firefox (windows only) to have an IE plugin so you can render something in IE's activeX control while in Firefox. But its not possible to have a simulator separate from IE's code show you what IE5.5, IE6, etc behaved.
Thats one of the reasons I have to use a virtual machine just to test IE6 (a fresh copy of XP SP2 comes with IE6 by default).