Firefox Personas is a new way of skinning the browser. This difference between Firefox Personas and normal themes is the fact that you don't have to restart Firefox to see the results.
To be able to use Firefox Persona, you will have to install the extension from the site Get Personas and restart Firefox afterwards. After restarting you will see a new button (a fox) at the bottom right of the window. Clicking on that button will reveal a menu allowing you to choose an array of categories.