You can use wmctrl to send a firefox window to below all other windows and maximise it (don't go full screen or it comes to the front). If you get your theming right you can "merge" the menubar with the browser window background.
Here are instructions and a script you could use to do such a thing:
open a text file and paste the following:
sudo apt-get install wmctrl
You need the sleep command to ensure that firefox loads up before the window commands try to run. they won't work if the firefox window isn't there!
wmctrl -r firefox -b add,maximized_vert,maximized_horz &
wmctrl -r firefox -b add,below &
If you wish you can create a new firefox profile just for this purpose, so your normal profile can operate in a smaller window.
Save this as "desktopbrowser.sh" somewhere you like and make executable.
Change the background in firefox to a wallpaper you like. To make it look "seamless" use the same wallpaper as your real desktop.
Add the script to your startup sessions list Applications>System>Preferences>Sessions (? I am on openbox!)
Log out and in again - your firefox should open, maximise and go below other windows. If you want icons, Try using the Fast Dial extension to set up clickable images on your new browser "desktop".
check out lovinglinux thread on making a media center out of firefox for more ideas and on how to setup fast dial