The new version sports a wide range of improvements over the first test version of the browser upgrade, most notably plugs for memory leaks, security fixes, and a download manager that includes improvements previously available only through plug-ins.
The security enhancements get fairly technical, but the Firefox developer's Web page states that the new version offers "protection from cross-site JSON data leaks, tighter restrictions on site-specific content using effective TLD service, better presentation of Web site identity and security, malware protection, stricter SSL error pages, antivirus integration in the download manager, (and) version checking for insecure plug-ins."
The updates to the download manager are pretty good, and they're far easier to parse. The new manager lets you resume stopped downloads, and it has the aforementioned built-in virus checker.
You can also zoom in on parts of a Web page, and the integration between bookmarks, the location bar, and bookmark folders has become tighter. There's now one-click bookmarking, smart folders for bookmarks, and the location bar checks against your bookmarks and history for page titles and URLs.
The Firefox developers also claim that Web site rendering is now faster than it was in FF3 beta 1, but empirically, I couldn't detect any difference. Either way, it's definitely faster than it is in FF2.
If you're a bit nervous about accidentally damaging your current installation of Firefox 2, but you'd like to try out the Firefox 3 beta, I recommend installing the portable version. It doesn't affect your old settings, though you can't run both Firefox 2 and Firefox 3 beta portable simultaneously.