If you would like to run these early-release versions of Firefox, you can download the Aurora channel here.
New Privacy Feature: Data Management Window
For users, the most important addition is likely the Data Management Window (type about:permissions into the URL bar to access it). This is currently an experimental feature and may not make it into the final version of Firefox 6. This new privacy tool gives you the ability to set fine-grained permissions for every site you visit. You can allow or disallow whether you want the site to write cookies, open pop-up windows, maintain offline storage and access your location, for example.
New Developer Tools
In addition to all of this, the Aurora channel now also features additional support for more HTML5 features and networking features. You can find a full list of these here.