Rated 4 out of 5 stars
You don't need to have administrator privileges to change your operating desktop. If you are a limited session user, desktop wallpaper can be changed. At least on Windows XP with Firefox 3.6.
The only good wallpaper in your gallery is Firefox logo. Others are awfully black.
Why cannot you have all options in one place? Why different windows for them?
Your add-on is overly complex. What do you use Ajax for?
My "home-made" add-on has neither a skin nor a language pack. It doesn't need them.