Hi This is not really a Firefox for Android question, but I will do my best to help. If you long press on the centre of the screen (away from any app icons), you should be presented with a menu that will allow you to set and change the desktop wallpaper. This and other display options will depend on the make and model of your mobile device, so if in doubt, I recommend speaking to the place where you bought the device, or on the manufactures website.