Earlier quoted context omitted.
Use IETab for those specific websites, seamless support.
Not seamless in my experience. There is one site in particular that I need to use whenever my domains need renewing; the javascript menus didn't work in the IE tab, but was just fine viewed from IE itself. Not a clue why.
Installed a fully secured suite of FF in a law office w/IETabs. Forms wouldn't load in it. Chaos ensued.
None of them ever used FF again and now tell everyone it's "that crappy program tech support installed".
Now I didn't uninstall IE but I hid the icon. Lesson learned. Just leave the icon in their quicklaunch with FF on the left of it. Instruct them to use the orange icon because it's safer and faster. If they don't use it and their computer gets fubar'd you can just tell them that you told them so and fix it up for them.