gecko-dev/browser/base/content/tabbrowser.css

.tabbrowser-tabbox {
  -moz-binding: url("chrome://browser/content/tabbrowser.xml#tabbrowser-tabbox");
}

.tabbrowser-arrowscrollbox {
  -moz-binding: url("chrome://browser/content/tabbrowser.xml#tabbrowser-arrowscrollbox");
}

.tab-close-button {
  -moz-binding: url("chrome://browser/content/tabbrowser.xml#tabbrowser-close-tab-button");
  display: none;
}

.tabbrowser-tabs:not([closebuttons="noclose"]):not([closebuttons="closeatend"]) > * > * > * .tab-close-button:not([pinned])[selected="true"],
.tabbrowser-tabs[closebuttons="alltabs"] > * > * > * > .tab-close-button:not([pinned]) {
  display: -moz-box;
}

.tab-close-button[selected="true"] {
  /* Make this button focusable so clicking on it will not focus the tab while
     it's getting closed */
  -moz-user-focus: normal;
}

.tab-label[pinned] {
  display: none;
}

.tab-stack {
  vertical-align: middle; /* for pinned tabs */
}

tabpanels {
  background-color: white;
}

.tab-drop-indicator {
  position: relative;
  z-index: 1;
}

/* tab progress */

.tab-progress {
  -moz-binding: url("chrome://global/content/bindings/progressmeter.xml#progressmeter") !important;
  margin: 0;
  min-width: 0;
}

.tab-progress[selected],
.tab-progress:not([busy]) {
  display: none;
}

.tab-progress-container {
  -moz-box-pack: start;
}