My Firefox userChrome.css file

/* =========================================================
   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; /* also reduces or increases the gap between rows */
  height: 39px !important;  /* also reduces or increases the gap between rows */
}

/* 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
   ========================================================= */

/* Active window color */
.browser-titlebar {
  color: #000000 !important;
  background-color: #dfdfe2 !important;
}

/* 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 BORDER RADIUS
   ========================================================= */

:root {
  --tab-border-radius: 8px !important;
}

/*
   TAB COLORS
   WHITE  = normal unselected tab
   ORANGE = unselected tab when mouse is hovering
   GREEN  = currently selected tab

   #FFFFFF = White
   #FF7F50 = Coral / Orange
   #66CDAA = MediumAquamarine
 */

/* Normal unselected tab = WHITE */
.tabbrowser-tab:not([selected]):not(:hover) .tab-background {
  background-color: #FFFFFF !important;
  background-image: none !important;
  border-radius: 8px 8px 0px 0px !important;
  box-shadow: 1px 0px 1px 0px rgba(0, 0, 0, 0.4) !important;
}

/* Unselected tab with mouse hover = ORANGE */
.tabbrowser-tab:not([selected]):hover .tab-background {
  background-color: #FF7F50 !important;
  background-image: none !important;
  border-radius: 8px 8px 0px 0px !important;
  box-shadow: 1px 0px 1px 0px rgba(0, 0, 0, 0.4) !important;
}

/* Selected current tab = GREEN */
.tab-background:is([selected], [multiselected]) {
  background-color: #66CDAA !important;
  background-image: none !important;
  border-radius: 8px 8px 0px 0px !important;
  box-shadow: 0px 1px 0px 1px rgba(0, 0, 0, 0.4) !important;
}

/* Extra force for selected current tab */

.tabbrowser-tab[selected="true"] .tab-background {
  background-color: #66CDAA !important;
  background-image: none !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;
}

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

/* Next go to about:config.Search for: browser.tabs.tabClipWidth
Change its value from the default 140 to 0 (or 1).
This tells Firefox never to strip away the close button based on width restrictions
*/