Firefox : Wallpaper your Firefox new tab page

1.After a restart, click on the Stylish icon in the add-ons toolbar (you can move it later to another toolbar or just access it through the Add-ons Manager), and select Manage Styles….
2.Click on Write a new style. Enter a name like My Custom New Tab Wallpaper
3.In the code section enter the following:

    /* Set a wallpaper for the about:newtab page*/
    @-moz-document url("about:newtab") {
    #newtab-scrollbox{
        background: url("file:///c:/machu.jpg") no-repeat !important ;
        background-size: 100% !important;
     }
    }

引用元: Wallpaper your Firefox new tab page • Mozilla Links
http://mozillalinks.org/2012/06/wallpaper-your-firefox-new-tab-page/