/* https://www.reddit.com/r/FirefoxCSS/comments/8ruy2d/numbered_tabs_revisited/ */
#tabbrowser-tabs{
  counter-reset: n_tabs 0;
}
.tabbrowser-tab .tab-content{padding-left: 2px;}
.tabbrowser-tab .tab-content::before{
  display: -moz-box;
  padding-right:4px;
  counter-increment: n_tabs;
  content: counter(n_tabs);
}