Firefox 4 Beta 2 is now available for download. The beta update includes some of these new features from the previous Beta 1:
The biggest feature of the update is the addition of "App Tabs" which open every time you open a browser window and take up less room than a tab. You also can't accidentally close them. This is great for things like webmail and streaming radio services that you might like to have off to the side while doing other browsing. When the feature is complete it will be persistent across browsing sessions.
Remember that all your add-ons still might not work right away. If you have something that you want the Firefox dev team to know about make sure you use the feedback button to the right of the navigation bar. A full set of release notes can be found on the Firefox 4 Beta 2 release notes page.