To remove a persona or theme from Firefox, click on the orange "Firefox" button in the corner of the window (or the "Tools" menu in some versions) and choose "Add-ons." Click on "Appearance" to see the list of themes, and click on "Default" to re-enable the default theme. To change the background picture on your computer's desktop, right-click on the desktop and choose "Personalize", or follow the directions on this web page: http://windows.microsoft.com/en-US/windows7/change-your-desktop-background-wallpaper