userChrome.css

@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"); 

/* Improve Findbar */
@keyframes scale-in {
    0% {
        transform: scaleY(0);
    }
    100% {
        transform: scaleY(1);
    }
}

@keyframes scale-out {
    0% {
        transform: scaleY(1);
    }
    100% {
        transform: scaleY(0);
    }
}

.browserContainer {
    position: relative;
}

findbar {
    animation: 0.2s scale-in;
    transform-origin: top center;
    background: var(--toolbar-bgcolor) !important;
    padding: 4px 6px 6px 6px;
    border: 1px solid var(--toolbox-border-bottom-color);
    border-top-width: 0;
    border-radius: 0 0 var(--toolbarbutton-border-radius) var(--toolbarbutton-border-radius);
    position: absolute;
    top: -2px;
    right: 38px;
}

findbar[hidden="true"] {
    animation: 0.2s scale-out;
}

findbar .findbar-container {
    padding: 0 !important;
    padding-inline-start: 0 !important;
    padding-inline-end: 0 !important;
}

findbar .findbar-container .findbar-textbox {
    padding: 6px 8px !important;
    background-color: var(--url-and-searchbar-background-color) !important;
    border-color: var(--backbutton-border-color) !important;
}

findbar .findbar-container .findbar-textbox[status="notfound"] {
    background-color: #E74C3C !important;
    color: white !important;
}

findbar .findbar-container .findbar-find-previous,
findbar .findbar-container .findbar-find-next {
    background: transparent !important;
    border-color: var(--backbutton-border-color) !important;
}

findbar .findbar-container .findbar-find-previous:hover, findbar .findbar-container .findbar-find-previous:focus,
findbar .findbar-container .findbar-find-next:hover,
findbar .findbar-container .findbar-find-next:focus {
    background-color: var(--toolbarbutton-hover-background) !important;
}

findbar .findbar-button {
    border: 1px solid var(--backbutton-border-color);
    border-radius: var(--toolbarbutton-border-radius) !important;
}

findbar .findbar-closebutton {
    margin-right: 4px !important;
    padding-inline-end: 0 !important;
    position: relative;
    top: 6px;
}

window[lwtheme="true"] findbar {
    background: var(--lwt-header-image) !important;
    background-attachment: fixed !important;
    background-position: top right !important;
    top: -1px;
}

window[lwtheme="true"] findbar:before {
    display: block;
    content: '';
    position: absolute;
    top: 0;
    left: 0;
    z-index: 0;
    width: 100%;
    height: 100%;
    background-color: var(--toolbar-bgcolor);
}

window[lwtheme="true"] findbar .findbar-container,
window[lwtheme="true"] findbar .findbar-closebutton {
    position: relative;
    z-index: 1;
}

window[lwtheme="true"] findbar .findbar-textbox {
    color: var(--lwt-text-color) !important;
    background-color: var(--toolbar-bgcolor) !important;
}

window[lwtheme="true"] findbar .findbar-textbox[status="notfound"] {
    background-color: #E74C3C !important;
    color: white !important;
}

window[lwtheme="true"] findbar .findbar-button[checked="true"] {
    font-weight: bold;
    background-color: var(--toolbar-bgcolor) !important;
}

window[lwtheme="true"] findbar .findbar-find-status, window[lwtheme="true"] findbar .found-matches {
    color: var(--lwt-text-color) !important;
}

findbar .textbox-input-box {
    margin-left: 12px !important;
}

/* Color private browsing window */
#main-window[privatebrowsingmode="temporary"] #TabsToolbar {
  background-color: #25003E !important;
  -moz-appearance: none !important;
}

/* Keep background tabs visible */
#main-window[privatebrowsingmode="temporary"] .tab-background:not([selected]) {
    background-color: var(--toolbar-bgcolor) !important;
}

/*
 * Bookmarks toolbar is visible only on new tab page, just like Chrome.
 * source: https://github.com/Timvde/UserChrome-Tweaks/blob/master/toolbars/show-bookmarks-only-on-newtab.css
 */

#main-window #PersonalToolbar {
  visibility: collapse !important;
}

#main-window[title^="about:newtab"] #PersonalToolbar,
#main-window[title^="New Tab"] #PersonalToolbar,
#main-window[title^="Nightly"] #PersonalToolbar,
#main-window[title^="Mozilla Firefox"] #PersonalToolbar,
#main-window[title^="Firefox"] #PersonalToolbar {
  visibility: visible !important;
}