Re: Firefox wallpaper
Firefox saves it in the C:\Windows directory, and calls it 'Firefox Wallpaper'.
Going to save another picture in the same way using this method though, will wipe the other one, replacing it with the new one. I guess after finding it, you can rename this picture, so the new one will still be called 'Firefox Wallpaper'
I found out this by setting a picture as a background. Then right clicking on the desktop, going to Properties, and went to the Wallpaper tab, and it was placed there. After finding out what it was called I just went to Search in Windows, and searched for 'Firefox wallpaper', and got that result in the Windows Directory
Hope I've helped