Users running Firefox 3.5 or 3.6 can download the latest version by selecting "Check for updates" from the Help menu. While users running Firefox 4 RC, need not download anything as Mozilla deems that build identical to the final. 12 new features in Mozilla Firefox 4
D-day is here for Mozilla Firefox 4, almost 8 months after its first beta release. The launch comes eight days after Microsoft IE 9's official launch. Firefox 4 was originally scheduled to launch in November 2010. Damon Sicore, Mozilla's senior director of platform engineering, said in a message on a company forum, “Today's triage session concluded with all systems go for a Firefox 4 launch on March 22.”

http://www.mozilla.com/products/download.html?product=firefox-4.0rc2&os=win&lang=en-US