The newer version of Firefox from Mozilla is definitely better than Firefox 3.6 and is definitely worth trying out. Firefox 4 runs on the Gecko 2.0 web platform. Earlier today I downloaded Firefox 4 and put it through some drills, here is a review of Firefox 4 Beta 1 where you can find the new features and also the how Firefox 4 fares against other browsers in web browser benchmarks.
New User Interface:
Firefox 4 sports a completely different interface, some of which are from Opera and Google Chrome. Firefox 4 has some new buttons and also a unified menu which is similar to the one Opera 10.6 has. In addition to that the tabs also default to the top which is similar to Google Chrome.