My Firefox userChrome.css file

/* =========================================================
   TAB BORDER RADIUS & GLOBAL VARIABLES
   ========================================================= */

:root {
  --tab-border-radius: 8px !important;
  --lwt-selected-tab-background-color: #66CDAA !important;
}

#tabbrowser-tabs,
.tabbrowser-tab {
  --tab-selected-bg-color: #66CDAA !important;
  --tab-background-color: #66CDAA !important;
  --lwt-selected-tab-background-color: #66CDAA !important;
  --toolbar-bgcolor: #66CDAA !important;
}

/* =========================================================
   BOOKMARKS / LIBRARY
   ========================================================= */

/* Font size in Bookmarks > Manage Bookmarks > Library
   popup on the right-hand side */
@-moz-document url(chrome://browser/content/places/places.xhtml) {
  #placeContent treechildren {
    font-size: 14px !important;
    font-family: 'Trebuchet MS' !important;
  }
}

/* Font size in Bookmarks > Manage Bookmarks > Library
   popup on the left-hand side */
@-moz-document url(chrome://browser/content/places/places.xhtml) {
  #placesView {
    font-size: 14px !important;
    font-family: 'Trebuchet MS' !important;
  }
}

/* =========================================================
   MULTI-ROW TABS
   ========================================================= */

/* Allow tabs to wrap onto multiple rows and collapse vertical gaps */
scrollbox[part][orient="horizontal"] > * {
  flex-wrap: wrap !important;
  align-content: flex-start !important;
}

/* Remove vertical spacing between tab rows */
.tabbrowser-tab {
  margin-top: 0px !important;
  margin-bottom: 0px !important;
  padding-top: 0px !important;
  padding-bottom: 0px !important;
}

/* Tab height */
.tabbrowser-tab,
.tabbrowser-tab > .tab-stack {
  min-height: 39px !important;
  height: 39px !important;
}

/* Keep tab contents flush with the tab */
.tabbrowser-tab .tab-content {
  margin-top: 0px !important;
  margin-bottom: 0px !important;
  padding-top: 0px !important;
  padding-bottom: 0px !important;
}

/* Remove spacing between tab rows */
#tabbrowser-tabs {
  --tab-block-margin: 0px !important;
}

/* =========================================================
   TOOLBAR ORDER
   ========================================================= */

#nav-bar {
  order: 1 !important;
}

#PersonalToolbar {
  order: 2 !important;
}

#TabsToolbar {
  order: 3 !important;
}

/* =========================================================
   URL FIELD
   ========================================================= */

/* URL field text size */
#nav-bar {
  font-size: 12px !important;
}

/* =========================================================
   SIDEBAR
   ========================================================= */

/* Sidebar color / font size formatting */
.sidebar-placesTree treechildren::-moz-tree-cell-text {
  font-size: 15px !important;
  color: #000000 !important;
  font-family: 'Trebuchet MS' !important;
}

/* =========================================================
   BOOKMARK FOLDER ICON
   ========================================================= */

/* Bookmark folder code from sifferedd
   Folder-item.png file is from:
   https://github.com/Aris-t2/CustomCSSforFx/blob/4.4.8/current/image/folder-item.png

   Need folder file to be in the profile/Chrome folder.
*/

.bookmark-item[container],
treechildren::-moz-tree-image(container),
.folder-icon {
  list-style-image: url("folder-item4.png") !important;
}

/* =========================================================
   TITLE BAR
   ========================================================= */

/* Both active and inactive titlebar use the same color */
.browser-titlebar {
  color: #000000 !important;
  background-color: #dfdfe2 !important;
}

/* =========================================================
   SIDEBAR HOVER / SELECTED COLOR
   ========================================================= */

/* MediumAquamarine - green background on hover
   on sidebar or selected */
.sidebar-placesTree treechildren::-moz-tree-row(hover),
.sidebar-placesTree treechildren::-moz-tree-row(selected) {
  background-color: #66CDAA !important;
}

/* =========================================================
   NAVIGATOR TOOLBOX
   ========================================================= */

/* Code from sifferedd */
#navigator-toolbox {
  font-family: "Quicksand Medium" !important;
  color: #000000 !important;
}

.bookmark-item > .toolbarbutton-text {
  font-size: 14px !important;
  font-family: 'Quicksand Medium' !important;
}

#personal-bookmarks {
  font-size: 14px !important;
  font-family: 'Quicksand Medium' !important;
}

#bookmarksMenuPopup,
.bookmark-item {
  font-size: 14px !important;
  font-family: 'Quicksand Medium' !important;
}

/* =========================================================
   MAIN MENU FONT
   ========================================================= */

/* File, Edit, View, etc. font size */
#main-menubar {
  font-size: 15px !important;
  font-family: 'Quicksand Medium' !important;
}

/* =========================================================
   TAB COLORS
   ========================================================= */

/* Strip default Firefox top line & outline variables from container */
#tabbrowser-tabs,
.tabbrowser-tab {
  --tab-line-color: transparent !important;
  --lwt-tab-line-color: transparent !important;
  --tab-outline-color: transparent !important;
}

/* Normal unselected tab = WHITE */
.tabbrowser-tab:not([selected]):not([multiselected]):not(:hover) .tab-background {
  background-color: #FFFFFF !important;
  background-image: none !important;
  border-radius: 8px 8px 0 0 !important;
  box-shadow: none !important;
}

/* Unselected tab with mouse hover = ORANGE */
.tabbrowser-tab:not([selected]):not([multiselected]):hover .tab-background {
  background-color: #FF7F50 !important;
  background-image: none !important;
  border-radius: 8px 8px 0 0 !important;
  box-shadow: none !important;
}

/* Selected current tab = GREEN (without gray top border or top outline) */
.tabbrowser-tab[selected] .tab-background,
.tabbrowser-tab[multiselected] .tab-background {
  background-color: #66CDAA !important;
  background-image: none !important;
  border-radius: 8px 8px 0 0 !important;
  border: none !important;
  border-top: 0px solid transparent !important;
  outline: none !important;
  box-shadow: none !important;
  opacity: 1 !important;
}

/* Ensure inner tab elements do not render top line shadows */
.tabbrowser-tab[selected] .tab-background *,
.tabbrowser-tab[multiselected] .tab-background * {
  background-color: #66CDAA !important;
  background-image: none !important;
  border: none !important;
  outline: none !important;
  box-shadow: none !important;
}

/* Hide line overlays and pseudo-elements above the tab stack */
.tab-line,
.tab-background::before,
.tab-background::after,
.tab-context-line {
  display: none !important;
  background-color: transparent !important;
  border: none !important;
}

/* Selected tab inner background = GREEN */
.tabbrowser-tab[selected="true"] > .tab-stack > .tab-background > .tab-background-inner,
.tabbrowser-tab[multiselected="true"] > .tab-stack > .tab-background > .tab-background-inner {
  background: #66CDAA !important;
  background-image: none !important;
  opacity: 1 !important;
  box-shadow: none !important;
}

/* Remove default top line element inside tab stack */
.tab-line,
.tab-background::before,
.tab-background::after {
  display: none !important;
  background-color: transparent !important;
}

/* Selected tab inner background = GREEN */
.tabbrowser-tab[selected="true"] > .tab-stack > .tab-background > .tab-background-inner,
.tabbrowser-tab[multiselected="true"] > .tab-stack > .tab-background > .tab-background-inner {
  background: #66CDAA !important;
  background-image: none !important;
  opacity: 1 !important;
}

/* =========================================================
   TAB FONT
   ========================================================= */

/* Font size on tabs */
.tab-label {
  font-size: 12px !important;
  font-family: 'Quicksand Medium' !important;
  font-weight: bold;
}

/* =========================================================
   RIGHT-CLICK MENU / CONTEXT MENU
   ========================================================= */

/* Font size for right-click menu */
menupopup {
  font-size: 13px !important;
}

/* Font size for dropdown menu lines before bookmarks */
menubar {
  font-size: 14px !important;
}

/* =========================================================
   CONTEXT MENU COLORS
   ========================================================= */

menupopup:not(
  .in-menulist,
  .toolbar-menupopup,
  .toolbar-menupopup menupopup
) {
  --panel-background: #f9f9fb !important;
  --panel-color: #000000 !important;
  --panel-padding: 6px !important;
  --panel-border-radius: 12px !important;
  --panel-border-color: transparent !important;
}

/* =========================================================
   MENU HIGHLIGHT TEXT
   ========================================================= */

[_moz-menuactive]:not([disabled="true"]) {
  color: #000000 !important;
  font-weight: bold;
}

/* Disabled highlighted menu item */
[_moz-menuactive="true"][disabled="true"] {
  background-color: #ffffff !important;
}

/* =========================================================
   MENU HOVER COLOR
   ========================================================= */

menu:where(
  [_moz-menuactive="true"]:not([disabled="true"])
),
menuitem:where(
  [_moz-menuactive="true"]:not([disabled="true"])
) {
  background-color: #66CDAA !important;
  color: #000000 !important;
}

/* =========================================================
   BOOKMARK / PLACES TREE
   ========================================================= */

/* Selected and Focus */
#placesView treechildren::-moz-tree-row(selected),
#placesView treechildren::-moz-tree-row(selected, focus) {
  background-color: green !important;
}

/* Hovered row */
#placesView treechildren::-moz-tree-row(hover) {
  background-color: #66CDAA !important;
}

/* Hovered text */
#placesView treechildren::-moz-tree-cell-text(hover) {
  color: black !important;
}

/* =========================================================
   TAB CLOSE BUTTON
   ========================================================= */

/* Always display the close button on all tabs, regardless of size */
.tab-close-button:not([pinned="true"]) {
  display: -moz-box !important;
}

/* =========================================================
   TAB SPACING
   ========================================================= */

.tab-background {
	--tab-margin-block: 0 !important;
}
#tabbrowser-tabs[orient="horizontal"] {
	--tabstrip-min-height: 0 !important;
	padding-block-start: 5px; /* Adds space above tabs, remove if unwanted */
}

slot[part="items-wrapper"] { gap: 5px 0 !important; }