After 12 betas and two release candidates slowly let loose over the course of eight months, Firefox 4 has gone gold at last. You may grab the final, ready-to-go Firefox 4.0 release right now from Mozilla's website. The browser is available for Windows, Mac OS X, and Linux in a variety of languages.
If you need to be talked into grabbing the new release, Mozilla has put up a series of short video tours designed to highlight Firefox 4's many other new features, like bookmark synchronization across multiple systems and "Personas," which adorn the top of the browser's interface like Google's Chrome themes. I'd recommend at least giving Firefox 4 a shot, though, if only to see just how much work has gone into this release.