Even though Firefox gains a lot of users nowadays, there were still some sites who doesn't like or became inaccessible when viewed using other browser than IE. Check this list and you will be quite surprised, since some of them are big sites. I think they should start working to make their site browser-friendly by removing the text "We only support Internet Explorer".
What's the point of only supporting a single browser? People around the world has their own browser preferences and some have known that Opera, Firefox, Konqueror, or Safari works best (or better) than IE in most cases, so they choose that browser. If a site only accept IE, then they are wasting lots of customer who didn't have IE on their computer (mostly are from Linux people). If others can switch, why wouldn't they?