[mannigfaltig]

I've added a modification that removes any margin at the top:

    #TabsToolbar, #sidebar-header {
      visibility: collapse !important;
    }
    
    #TabsToolbar {
      margin-bottom: -21px !important;
    }

Then go to the hamburger menu > Customization and check "Title Bar" at the bottom left. Then it pretty much looks like FF 56.

I've also changed the style of TSTs in the add-on settings (all the way at the bottom; not in userChrome.css) such that the tabs don't take up as much vertical space and to hide the close box button (and only show it on the mouse-over event):

    .closebox {
      display: none
    }
    
    .tab:hover .closebox {
      display: block;
    }
    
    .tab {
      padding-top: 0px;
      padding-bottom: 0px;
    }

You can make `.tab` even tighter:

    .tab .icon {
      padding-top: 2px; /* reducing it by 2px */
      padding-bottom: 2px;
    }
    
    :root {
      --tab-height: 22px; /* reducing it by 4px */
    }