Firefox has the huge collection of add-on s . Some are really cool ones. Personas are the skins for firefox , Its not exactly the theme, but the skin.
When you install theme for firefox, the button styles, layouts etc gets changed . But when you install persona, its just the background image and the color which changes.
Open the website given below. You will find the huge collection of personas. If you want to try and see any skin, just hover the mouse pointer over the skin , and your browser temporarily wears the skin! . If you wanna select one, just click on it.