Phil Wilson has a nice writeup on Enhancing Firefox GUI usability where he points out one good thing about IE: a bigger Back button. Of course, Phil isn't one to jibe at Firefox's back button. Rather he goes on to teach us how remedy this usability flaw by making the Back button bigger in Firefox, and this is possible thanks to the customizability of Firefox.
All you need to do is to add a few lines to the userChrome.css file in your profile - Phil explains all. (Trivia: you can combine the 2 CSS rules into a single "padding: 0 10px;").