Comment by alinspired

Here is how i hide top tab bar with TreeStyleTab

  $ cat userChrome.css 
  /* Hide horizontal tabs at the top of the window */
  #main-window[tabsintitlebar="true"]:not([extradragspace="true"]) #TabsToolbar > .toolbar-items {
    opacity: 0;
    pointer-events: none;
  }
  #main-window:not([tabsintitlebar="true"]) #TabsToolbar {
      visibility: collapse !important;
  }
  
  /* Hide the "Tree Style Tab" header at the top of the sidebar */
  #sidebar-box[sidebarcommand="treestyletab_piro_sakura_ne_jp-sidebar-action"] #sidebar-header {
    display: none;
  }