With the latest IE version, it really isn't all that different. That being said, there are a few small differences. FF is supposedly safer, although I don't have any evidence to prove so. As for visual differences, they are generally minor. The push by Jacob for people to use FF is probably becuase it may take extra work on his part to make sure the site looks right on different programs and hes probably most used to FF.
An example of FF vs IE:
I am under the impression that Jacob has a lot of things he would like to do with fakku and having to spend his time on annoying little things like that could suck.