Bad idea blocking Internet Explorer. At school, only the teacher computers and the Macs have Firefox. Other than that, all of the computers use Internet Explorer. What if your site had some useful info that people would be looking up? They wouldn't be able to access it. Besides, I could always change the useragent so that IE would report it's Firefox (you can do the same with Firefox, or any other browser) , or I could use Proxify. Otherwise, if you want people to use Firefox, the least you could do is on every page or on certain pages make a "Get Firefox" image/link appear.