userChrome.css

:root {
--col_bg: #121212;
--col_fg: #ffda70;
--bg_image: url('$Insert your own image here$');
--radius_main: 10px;
--margin_main: 10px;
--shadow_main: 3px 3px 5px 0px rgba(0,0,0,0.6);

}

#TabsToolbar, #sidebar-header, .sidebar-splitter, #back-button, #forward-button {
    visibility: collapse;
}

#sidebar-box {
    margin-top: 15px;
    margin-left: var(--margin_main);
    margin-bottom: var(--margin_main);
}

#appcontent {
    margin: var(--margin_main);
}

#nav-bar-customization-target {
    padding-left: 50px;
}

#nav-bar {
    margin-top: var(--margin_main);
    margin-left: var(--margin_main);
    margin-right: var(--margin_main);
}

:root:not([uidensity="compact"]) #PanelUI-button {
    padding-inline-start: 5px;
    display: block;
    position: absolute;
    left: 0px;
    top: 5px;
    border-inline-start: 0px solid black !important;
}

body {
    background-image: var(--bg_image);
    background-repeat: no-repeat;
    background-size: cover;
}

.browserStack, #sidebar, #sidebar-box,
#appcontent, #tabbrowser-tabbox, #tabbrowser-tabpanels, .browserSidebarContainer,
.browserContainer, browser, #nav-bar {
    border-radius: var(--radius_main);
    box-shadow: var(--shadow_main) !important;
}

#navigator-toolbox {
    border-bottom: 0px solid black !important;
    margin-left: 286px;
    z-index: 1;
}

#browser {
    margin-top: -50px;
}

#appcontent {
    margin-top: 60px;
}