Simple.com (BankSimple) doesn't support my browser

I've never seen a new company not support so many newer browsers (not newest)...I have FireFox 3 on my Macbook Pro and PC, and IE 8 on my desktop and in either of them you can't sign in using these browsers. They expect FireFox version 5+, IE9+, or the most recent version of Chrome or Safari. How can they expect all of their users to have the most recent version of all the latest browsers? I can understand not supporting IE6 any longer, which I don't, but IE 8? FireFox 3? These browsers aren't THAT old, or THAT different than the newest versions, unless they're using 100% HTML5 in their app