Easy, change the userChrome.css to this:

    $ cat ~/.mozilla/firefox/$profile.default/chrome/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;
    }