Personas are slightly different than Firefox themes, which also change Firefox's navigation buttons, in that Personas leave the navigation buttons intact. Also, you don't have to restart your browser to add or change Personas, which you have to do with themes. Personas are certainly a nice extra, but they're far from groundbreaking.

You can find Personas by selecting Tools --> Add-Ons --> Themes and clicking Get Themes. (Yes, you get Personas by clicking Get Themes, which is quite confusing.)

When you install a second Persona, two buttons appear near the top of the browser: Undo and Manage Themes. Click Undo to revert to your previous Persona. Click Manage Themes and you'll be able to see all of your installed Personas, switch among them, and uninstall any you want.

Users of previous versions of Firefox were already able to use Personas, but only with an extension designed for that purpose. Now, with 3.6, the extension isn't required. However, the extension does make it easier to install, use, and manage Personas. Among other capabilities, it puts a small icon on the lower-left portion of your screen that lets you instantly switch themes and browser and install new ones.