Assuming everyone is using Firefox 1.5 by now and can't stand Internet Explorer - Here's a good alternative to checking browser compatibility over IE, Firefox and Netscape from one browser - Just use the current version of Netscape (8.4).
Netscape has all the usual tabbed browsing of Firefox, but... it also has the ability to switch "browser modes" between a "Firefox View" and an "Internet Explorer View" while a page is loaded. They also claim better security over IE (yea?) and many other features.
Handy as a secondary browser!
Info and download is here:
http://browser.netscape.com/ns8/