Firefox CSS tabs toolbar auto hide.

userChrome.css

/* tab bar auto hide */
@-moz-document url(chrome://browser/content/browser.xul),
               url(chrome://browser/content/browser.xhtml) {

  /* show tabs toolbar when navigator-toolbox gets hovered */
  #main-window:not([customizing]) #navigator-toolbox:hover #TabsToolbar {
	  visibility: visible !important;
	  opacity: 1 !important;
	  margin-bottom: 0px;
	  transition: visibility 0.1s linear, opacity 0.1s linear, margin-bottom 0.1s linear;
  }
  /* hide tabs toolbar when unfocused */
  #main-window:not([customizing]) #TabsToolbar:not(:hover) {
	  visibility: collapse !important;
	  opacity: 0 !important;
	  margin-bottom: -40px;
	  transition: visibility 0.1s linear, opacity 0.1s linear, margin-bottom 0.1s linear;
  }
}