Do you mean a persona?

• https://support.mozilla.org/kb/Using+themes+with+Firefox
• http://kb.mozillazine.org/Themes (Lightweight theme)

You can find more Personas on the www.getpersonas.com site.

No What I am looking for is the picture you see on your desk top - behind your logos. It comes along with upgrade 15.0.1

If your desktop wallpaper is lost then you need to find that previous image file and set it once again as your desktop wallpaper. Updating or (re)installing Firefox shouldn't have any effect on the current wallpaper, so if you lost it then something else went wrong.

• http://www.microsoft.com/resources/documentation/windows/xp/all/proddocs/en-us/display_change_desktop_background.mspx?mfr=true