There are many websites I am required to use for school, banking, work etc. that require IE. FF won't work and I tried for kicks and giggles. I have never been in a situation where FF was required and IE didn't work. From where I'm standing FF isn't worth the trouble since the only benefit I can see is that I could say that I use FF. Big whoop.
I have yet to stumble upon a site that was worth using and didn't work in both IE and FF. My bank's included.