I've got this menu working properly in FF. It doesn't work at all in IE. I understand that the transitions aren't supported in IE, but I expected the rest of it to work. I also understand that I need to do something with modernizr to polyfill for IE. But I don't know how to do this.
Please be patient with my ignorance. I'm pretty inexperienced with all of this. Thanks for your help!
The fix is really simple... you have a stray LI tag after the testimonials link. IE9 isn't kind enough to ignore it and that's why it's breaking.