My FireFox settings, just 4 u, genny

1. userChrome.css

/* Hide horizontal tabs at the top of the window #1349, #1672, #2147 */
#main-window[tabsintitlebar="true"]:not([extradragspace="true"]) #TabsToolbar > .toolbar-items {
   opacity: 0;
   pointer-events: none;
}
#main-window:not([tabsintitlebar="true"]) #TabsToolbar {
   visibility: collapse !important;
}
#TabsToolbar .titlebar-spacer {
   border-inline-end: none !important;
}
/* Hide horizontal tabs at the top of the window */
#tabbrowser-tabs {
   visibility: collapse !important;
}

/* 15px for dragging whole window by mouse*/
#titlebar {
   appearance: none !important;
   height: 22px; /* this number only works on windows */
}

/* Fix for main menu calling by Alt button */
#titlebar > #toolbar-menubar {
   margin-top: 10px;
}

/* Move minimize/restore/close buttons to empty space */
@supports -moz-bool-pref("layout.css.osx-font-smoothing.enabled"){
   #TabsToolbar > .titlebar-buttonbox-container {
      display: block;
      position: absolute;
      top: 34px;
      left: 0px;
   }
}

@supports not -moz-bool-pref("layout.css.osx-font-smoothing.enabled"){
   #TabsToolbar > .titlebar-buttonbox-container {
      display: block;
      position: absolute;
      top: 28px;
      left: 0px;
   }
}

/* Hide the "Tree Style Tab" header at the top of the sidebar (For only Tree Style Tab sidebar
 * #1397) */
#sidebar-box[sidebarcommand="treestyletab_piro_sakura_ne_jp-sidebar-action"] #sidebar-header {
   display: none;
}

/* Along with privacy.resistFingerprinting = true, this enforces 1366 × 768 internal dimensions.
 * See: <https://www.reddit.com/r/firefox/comments/8e0zzp/how_to_change_default_window_positionsize_when/> */
#sidebar {
   min-width: 234px !important;
   max-width: 234px !important;
}

/* Auto-hide sidebar in windows with only one tab (requires another extension) #1768 */
#main-window[titlepreface^="[1] "] #sidebar-box[sidebarcommand="treestyletab_piro_sakura_ne_jp-sidebar-action"] {
   visibility: collapse !important;
}

2. userContent.css

/*
   dark background in new tabs without a white flash (with tridactyl newtab)
   @see: https://github.com/tridactyl/tridactyl/issues/2510
*/
:root {
  --tridactyl-bg: #2e3440 !important;
  --tridactyl-fg: white !important;
}

/*
    set the background color of the new tab page (without tridactyl or with tridactyl without newtab)
*/
@-moz-document url-prefix(about:home), url-prefix(about:newtab) {
  body {
    background: #2e3440;
  }
}


/* Dark / Nord-theme formatting for DuckDuck go, to avoid pageflash */
@-moz-document domain("duckduckgo.com") {
  html,
  body,
  .body--home,
  .site-wrapper,
  .region__body,
  .badge-link,
  .module--carousel__image-wrapper,
  .result__image,
  .vertical--map__sidebar,
  .vertical--map__sidebar__header,
  .page-chrome_newtab,
  .zci--type--tiles:not(.is-fallback).is-full-page.is-expanded,
  .zci--type--tiles:not(.is-fallback).is-full-page.is-expanded
    .metabar:not(.is-stuck) {
    background-color: #2e3440 !important;
  }
}

3. treestyletab.css

/* THIS FILE MUST BE MANUALLY LOADED IN THE TST SETTINGS. */

/* Hide twisty and collapse/expand tree by clicking on tab's icon #1743 */
tab-item tab-twisty {
  margin-right: -0.7em;
  opacity: 0 !important;
  position: relative;
  z-index: 10000;
}

/* Grouping tabs with spaces instead of indented tree #2777 */
tab-item[data-level="0"] {
  margin-top: calc(var(--tab-size) / 3); /* please change this as you like */
}

tab-item[data-level="0"].pinned,
tab-item[data-level="0"].pinned + tab-item[data-level="0"],
tab-item[data-level="0"] + tab-item[data-level="0"]:not([data-child-ids]) {
  margin-top: 0;
}

/* Show title of unread tabs in italic #1363 */
tab-item.unread .label-content {
  font-style: italic;
}

#tabbar {
  counter-reset: vtabs atabs tabs;
  /* vtabs tracks visible tabs, atabs tracks active tabs, tabs tracks all tabs */
}
tab-item:not(.collapsed):not(.discarded) {
  counter-increment: vtabs atabs tabs;
}
tab-item:not(.collapsed) {
  counter-increment: vtabs tabs;
}
tab-item:not(.discarded) {
  counter-increment: atabs tabs;
}
tab-item {
  counter-increment: tabs;
}

/* Numbering of tabs #1601, #2220 */
tab-item .extra-items-container.behind {
  z-index: unset !important;
}
tab-item .extra-items-container.behind::after {
  background: Highlight;
  color: HighlightText;
  content: counter(vtabs);
  font-size: x-small;
  right: 0.2em;
  padding: 0.2em;
  pointer-events: none;
  position: absolute;
  bottom: 0.2em;

  z-index: 1000;
}

/* Hide the "new tab" button at the bottom edge of the tab bar #1591 */
.newtab-button-box {
  display: none;
}
#tabbar {
  bottom: 0 !important; /* Eliminate dead space on bottom */
}

/* "#tabbar" is required for TST 3.4.0 and later! */
#tabbar tab-item:not(:hover) tab-closebox {
  display: none;
}