I've just downloaded IE tab who is a firefox plugin and who should be a must for any webmaster using firefox.
When you open a page using firefox you can choose to open it with IE just by doing a right click or change the web search engine (and that is realy grand) from firefox you go to IE in a click, you can then compare your website looks from one to the other

http://ietab.mozdev.org/

that is a very usefull tool indeed and i didn't have a clue that it existed, I actualy never had even though of doing so when it now seems obvious once I think of it .....