- Download and install the IE Tab add-on (like all add-ons, this will require a restart of Firefox)
- Go to the site that you want to render each time in IE
- Open the IE Tab Options (Tools >> IE Tab Options -- Sites Filter tab)
- The page you just navigated to should be in the URL box, so you just need to click Add
You can skip step 2 and enter any URL manually into the URL box
- Click OK
There's one more trick related to this that may be required for some sites. Sites will typically look at the User Agent of a web browser -- this tells the site what browser you are using -- to determine how certain aspects should render or even whether it will display a page. Use the add-on, User Agent Switcher, you can tell a website that you have a browser that is different than the one you are actually using.
The particular site I use IE Tab with will not even let me login if the User Agent doesn't indicate Internet Explorer. So in addition to adding IE Tab to my browser, I had to add User Agent Switcher. Now any site I visit, by default it thinks my browser is Internet Explorer, even though it is really Firefox.