userChrome.css

/* To enable userChrome.css: Open `about:config` set `toolkit.legacyUserProfileCustomizations.stylesheets` to true */

/* Hide nav bar, source code form https://www.reddit.com/r/FirefoxCSS/comments/rcogwz/comment/hnzf5rg */
#nav-bar {
  /* customize this value. */
  --navbar-margin: -38px;

  margin-top: var(--navbar-margin);
  margin-bottom: 0;
  z-index: -100;
  transition: all 0.3s ease !important;
  opacity: 0;
}

#navigator-toolbox:focus-within > #nav-bar,
#navigator-toolbox:hover > #nav-bar {
  margin-top: 0;
  margin-bottom: var(--navbar-margin);
  z-index: 100;
  opacity: 1;
}

/* Add number on tab, source code from https://gist.github.com/arthurattwell/19d868330a3c9f668a3f3a69230ac28c */
tabs {
  counter-reset: tab-counter;
}

tab:nth-child(1) .tab-label::before,
tab:nth-child(2) .tab-label::before,
tab:nth-child(3) .tab-label::before,
tab:nth-child(4) .tab-label::before,
tab:nth-child(5) .tab-label::before,
tab:nth-child(6) .tab-label::before,
tab:nth-child(7) .tab-label::before,
tab:nth-child(8) .tab-label::before {
  background-color: white;
  border-radius: 0.25em;
  border: 1px solid white;
  box-sizing: border-box;
  color: black;
  content: counter(tab-counter) '';
  counter-increment: tab-counter;
  display: block;
  float: left;
  font-size: 0.8em;
  font-weight: bold;
  height: 1.5em;
  line-height: 1;
  margin: 0 0.5em 0 0;
  padding: 0.1em 0.25em 0.25em 0.25em;
  position: relative;
  text-align: center;
  top: 0.35em;
  vertical-align: middle;
  width: 1.4em;
}