The Firefox 5 beta is only a month away thanks to Mozilla's speedy new development cycle, but you needn't wait until then to try the new browser.
A new channel called Aurora gives Firefox users an even earlier look at Firefox 5. It's not as stable as the Firefox 5 beta, but it's more reliable than the browser's nightly builds. In that sense, it's kind of like Google Chrome's dev channel.
In a blog post, Mozilla's Christian Legnitto explained that the expectation of quality for Firefox's beta channel is considerably higher than it is for nightly builds. Aurora splits the difference, and allows Mozilla to work on features without creating a backlog of patches for the beta.
You can download Firefox Aurora here, but don't get too excited. None of the Firefox 5 features that we've heard about, such as hot links from pinned tabs and social network sharing, are in place yet, and add-ons are unlikely to work. Also, Aurora installs over whatever version of Firefox you're currently running, so you'll have to reinstall Firefox 4 if you want to go back to a stable build. (At least Aurora uses a different icon so you know which version you're running.)
If you're a Firefox user with a little more patience, you can wait for the Firefox 5 beta, which is should be coming May 17. Mozilla expects the final version of Firefox 5 to launch by June 21.