If you haven't tried updating past 3.6, you should. Benchmarks may not be 100% accurate, but you can see the difference in JS-heavy apps (like GMail). The UI has been cleaned up and revamped. There are a ton of new HTML/CSS/JS features. Better memory management, and Firefox as a whole is faster. Restartless add-ons. The list goes on and on :)

Oh I have upgraded, and there are things that I like about it. However the title "What you are missing" isn't really addressed by showing a bunch of benchmarks. Having a real-world section talking about subjective performance on actual sites used by people would have done that.