I've never looked back since switching to Firefox although I've never felt passionately enough to install a Firefox desktop wallpaper.
However, if you are a huge fan, then Firefox Wallpaper Pack features a collection of images celebrating the iconic browser. There are over 40 to choose from, mainly featuring the Firefox logo but some of them focusing more on the "Fire" in Firefox. The resolutions vary however meaning that they're not the best quality wallpapers out there and may not look great on particularly large screens.
If you're a die hard fan of Firefox, then you'll love this huge collection of images celebrating Mozilla's masterpiece.