Firefox has launched a newer feature called Personas that lets you personalize the look of your Firefox in simple one click.You won't have to download anything in order to change your Firefox theme 'coz these are available online and installs with just a click on the image of the theme you liked.You can also view the demo of the theme you are going to install by simply hovering your mouse on the theme's image available on the Personas website.

Here is a screenshot of my Firefox persona:

Click here to visit Official Firefox Persona Website.