Most designers know better than to design a site for a specific browser. The latest version of Dreamweaver MX (the Web design program I use) will check a design against a dozen or so different browsers, both Windoze and Mac, alert you if you've written incompatible code, and highlight the specific tags or whatever. I'm in the process of rewrtiting my HTML sites to be compaitble with XHTML, and I use Firefox as my main "look and feel" browswer because it holds XHTML sites to strict compliance. Frankly, if the site works with Firefox, it'll work with everything except Netscape 4.0 (which is the world's most incompatible browser).
If for no other reason, you should switch away from IE because of its numerous security holes. They are so severe that the US Department of Homeland Security has issued this warning
about them: "There are a number of significant vulnerabilities in technologies relating to the IE domain/zone security model, the DHTML object model, MIME type determination, the graphical user interface (GUI), and ActiveX. It is possible to reduce exposure to these vulnerabilities by using a different web browser, especially when browsing untrusted sites." Caveat surfer! --Randy