If you don't like Internet Explorer, and would rather use Firefox, some sites make it hard for you. Sometimes, they just nag you with warnings. Other times they just plain refuse to load in your browser, or even try to warn you your operating system is wrong.
When you come upon a naggy site that doesn't like your browser, go to tools, select User agent switcher, and select the browser you need to pretend to be.
Reload the page and the prejudice ends. You can use it on many sites to log-in to your classes or pay your bill. It will even fool Mozilla.org into insulting you for using IE. That's how good it is.
Do be aware, sometimes the reason they warn you to use a certain browser is because their crappy site BREAKS in every other browser, so the user agent won't help you in those situations. But for a lot of sites, you'll be able to use them just as if you had Internet Explorer running in Windows.