Using Firefox to access a site built for IE only? (shame on them!) just download and add user agent switcher addon to your Firefox and pretend to be IE. Be warned that the website may not function property if it uses non-standard and IE specific functions. In this case it’s best to use IE.
With this add-on, not only you can lie about the browser you’re using. You can lie about your OS too.
As a web developer I find this a useful tool to test browser specific code without the need to switch to a different browser or even install all the different browsers!
You can import a list of user agents here. I believe the list is constantly updated.