tree-style tab setup

How to get rid of the top bar in firefox after installing tree style tab:

In about:config (accept the risk), search toolkit.legacyUserProfileCustomizations.stylesheets and hit the funny looking button to toggle it to true.

In about:support, above the fold in the “Application Basics” section, find Profile Directory.

In that directory, mkdir chrome, then create userChrome.css, containing:

#main-window[tabsintitlebar="true"]:not([extradragspace="true"]) #TabsToolbar
  opacity: 0;
  pointer-events: none;
#main-window:not([tabsintitlebar="true"]) #TabsToolbar {
    visibility: collapse !important;