What you need is Firefox 3.6 and above (in order to use Firefox Personas), a personal iGoogle page, or at least a google account and some wallpapersr which can be found here, of course.
Theme number One: Lord of the Rings
get the Firefox perosna One Ring to Rule Them All
Download the desktop backgrounds below.
Theme number Two: The Raindrop
get the Firefox persona Raindrop Green
Click the image to save the wallpaper.
Theme number Three: The Ultimate Theme -Pink Sunset
Firefox Persona Purple Pines Sunset
You can find the iGoogle theme here.
Click and download the wallpaper below
If you are using Vista or Windows 7 or even Ubuntu 10.04 you can also change the color of the window borders and the taskbar.