Minimalistic Firefox Nightly

userChrome.css

#main-window {
    min-width: 400px !important;
}

/* Hide the shield (tracking protection) icon */
#tracking-protection-icon-container {
    display: none !important;
}

/* Hide site information (lock icon) */
#identity-box {
    display: none !important;
}

/* Hide search engine selector */
#urlbar-searchmode-switcher {
    display: none !important;
}

/* Add back the padding for the URL bar */
moz-input-box.urlbar-input-box {
    padding-inline-start: var(--urlbar-icon-padding) !important;
}

/* Remove spacers */
hbox.titlebar-spacer[type="pre-tabs"], hbox.titlebar-spacer[type="post-tabs"] {
    display: none !important;
}

toolbarspring#vertical-spacer.chromeclass-toolbar-additional.titlebar-spacer {
    flex: 0 !important;
}

/* Hide the favorites (bookmark) star */
#star-button-box {
    display: none !important;
}

/* Hide the extensions menu */
#unified-extensions-button {
    display: none !important;
}

/* Hide the navbar overflow button */
#nav-bar-overflow-button {
    display: none !important;
}

#page-action-buttons {
    display: none !important;
}

#navigator-toolbox {
    border-bottom: none !important;
}

#browser {
    background: black !important;
}

@media (-moz-bool-pref: "sidebar.revamp") {
  #sidebar-main, #sidebar-box {
    background-image: unset !important;
  }
}

#tabbrowser-tabbox {
    outline: solid 1px #222 !important;
}

:root {
    --newtab-background-color: red;
}

/* Bump up the URL font size */
#urlbar-input {
    font-size: 1.125em !important;
}

@media (max-width: 500px) {
    toolbarspring#vertical-spacer.chromeclass-toolbar-additional.titlebar-spacer,
    toolbarbutton#sidebar-button,
    toolbarbutton#back-button,
    toolbarbutton#forward-button,
    toolbaritem#PanelUI-button
    {
        display: none !important;
    }
}

userContent.css

body {
  --newtab-wallpaper-color: black !important;
}

.search-wrapper .search-handoff-button {
  --newtab-background-color-secondary: #1e1e1e !important;
}