Why IE Only?

I'm no longer using IE do to Security Issues. I'm using Firefox as my main browser now and am very happy with it I will never switch back to IE unless I need updates. But still alot of people and websites insist on using IE. Why? I don't know. But I needed to get on an important page for school and noticed that it was only IE 5.5 or better compatible (Screenshot Below)

What are the people using OSX or Linux going to do? IE For OSX is long outdated so there just screwed. I really hate and don't use sites like this. NOT EVERYONE CAN USE IE. I avoided these sites if it's not important. There is a little extension for FF to open pages in IE but it still has to launch IE and then it brings FF to a crawl. So if you are designing a website make sure you use multi-platform support because users of Mac's Linux Boxes and People that use FF won't be able to use it. Remember the numbers are going up for FF and other browsers.