How do you change the background on Firefox 8?

To change the background on Firefox 8, you can use a custom user style through the "Stylish" extension or apply a theme. Install the Stylish add-on from the Firefox Add-ons site, create a new style, and use CSS to define your desired background. Alternatively, you can browse and install pre-made themes directly from the Add-ons site to change the overall look, including backgrounds, of your Firefox browser.