For different versions you'll need to either wait (or know how to navigate around an FTP server (... ftp://releases.mozilla.org/pub/mozilla.org/firefox/releases/4.0/ ... you didn't get this from me ... ;).
Lots of new stuff in this release, including:
Newly redesigned user interface
Built-in sync feature
Support for HTML5, CSS3 and WebGL
I do need to pass on a word of caution though. This from Mike Beltzner, Mozilla's director of Firefox:
As part of Mozilla's open source process we post our builds publicly for testing and verification. These builds are part of our open development process. While we appreciate everyone's excitement about the Firefox 4 release, these builds are currently NOT FINAL, and are still going through QA testing. Firefox 4 is expected to launch tomorrow morning as the QA process is complete and the first run experience and interactive launch events are ready. In the meantime, we would prefer that people download and test the Firefox 4 release candidate from: www.mozilla.com/firefox/rc
I've downloaded and installed the code on a number of systems and it seems to me to be the final release (neither I nor my PCs have burst into flames or anything). The 'first run experience' page still says RC but that will change tomorrow.