Here are three themes that would make your desktop seemlesly match with firefox and iGoogle or just Google.
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

Screenshots:

get the Firefox perosna One Ring to Rule Them All
Download the desktop backgrounds below.

 And you can use these as Google Backgrounds too. (Change Background>From My Computer>Browse). I recommend the second wallpaper as  google background.

Theme number Two: The Raindrop

 Screenshots:

get the Firefox persona Raindrop Green

In order the change the Google background image, you can use the one that you downloaded here, or at google home page, go to Change Background>Public Gallery and in the Search box type: “raindrops in green by silfiriel” (without quote marks), and you can select the image.

Theme number Three: The Ultimate Theme -Pink Sunset

Screenshots:

Firefox Persona Purple Pines Sunset
You can find the iGoogle theme here.
Click and download the wallpaper below

You can use this image as a Google background by uploading it. (Change Backgound>From My Computer>Browse)

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.