usercontent.css

@-moz-document url("about:home"), url("about:newtab") {
    /* 1. Makes the entire new tab background transparent */
    body, #root, #root > div, .outer-wrapper {
        background: transparent !important;
        background-color: transparent !important;
    }

    /* 2. Completely hides the default Firefox logo and branding text */
    .logo-and-wordmark-wrapper, .logo-and-wordmark, .logo, .wordmark {
        display: none !important;
    }

    /* 3. Sets up the search container and pushes it down to its original spot */
    .search-wrapper {
        position: relative !important;
        margin-top: 160px !important; /* Pushes the search bar down to where it originally lived */
    }

    /* 4. Injects the star right above the search bar, perfectly centered */
    .search-wrapper::before {
        content: "";
        position: absolute;
        left: 50%;
        transform: translateX(-50%);
        top: -120px;                 /* hovers perfectly right above the search input field */
        width: 100px;
        height: 100px;
        background: url("./my-logo.gif") no-repeat center/contain !important;
    }
}