For a kiosk setup, it can be a bit disruptive for users to be changing the Windows desktop wallpaper. They might change it to something offensive, hard to read, or simply unprofessional. This function can be disabled for most applications through group policy, but Firefox still allows a hole.
In Firefox, if you right-click on an image the context menu offers ‘Set as Desktop Background’ which somehow gets around the GPO block and lack of access to the Display properties in the Control Panel.
Using a Firefox extension, Menu Editor, you can simply remove this option from the right-click menu.
Install Menu Editor and then go into the add-on’s Options. You can scroll through the list and choose things you want to hide or re-arrange so your context menu suits you best.
With the kiosk setup, if somebody really wants to set the wallpaper they simply have to uninstall the Menu Editor add-on. Then all the options you removed are available again. Another add-on can resolve this problem: Public Fox. Public Fox allows you to password protect a lot of settings, including about:config, Add-Ons, Options, and more. You can also black-list sites to prevent access to particular websites.
Password protect the Add-ons so Menu Editor and other extensions do not get uninstalled.