Hide Firefox tabs bar completely (for usage with vertical tabs)

userChrome.css

#main-window[tabsintitlebar="true"]:not([extradragspace="true"]) #TabsToolbar > .toolbar-items {
  opacity: 0;
  pointer-events: none;
}

#main-window:not([tabsintitlebar="true"]) #TabsToolbar {
  visibility: collapse !important;
}

#nav-bar {
  /* Add some space for window controls */
  /* Button width is 45px so it's 45px * 3 + 5px */
  margin-right: 140px;
}

/* Some space on top of the window to allow window dragging */
#titlebar {
  appearance: none !important;
  height: 5px;
}

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

/* Window controls container */
.titlebar-buttonbox-container {
  display: block;
  height: 50px; /* TWEAKME */
  margin-top: -5px; /* the same as #titlebar height */
}

.titlebar-buttonbox-container .titlebar-button {
  display: block;
  height: 45px; /* TWEAKME */
}

/* Hide sidebar header */
#sidebar-box[sidebarcommand="treestyletab_piro_sakura_ne_jp-sidebar-action"] #sidebar-header {
  display: none;
}