Tab close button animation

.tabbrowser-tab .tab-close-button .toolbarbutton-icon {
  width: 0;
}

.tabbrowser-tab:not(:hover) .tab-close-button {
  width: 0;
  transition: width .2s;
}

.tabbrowser-tab:hover .tab-close-button {
  width: 16px;
  transition: width .2s;
}