Firefox about:newtab background image

userContent.css

/*
    1. Set toolkit.legacyUserProfileCustomizations.stylesheets to true in about:config
    2. Place this file (userContent.css) in the chrome directory of your firefox profile
    3. Place an image *hello.jpg* in the same directory as this file
*/

@-moz-document url(about:home), url(about:newtab), url(about:privatebrowsing) {
    body::before {
        content: "";
        z-index: -1;
        position: fixed;
        top: 0;
        left: 0;
        background: url(hello.jpg) center;
        background-size: cover;
        width: 100vw;
        height: 100vh;
    }
}