fenix-fox/chrome/tab_close_button_always_on_hover.css

/* Source file https://github.com/MrOtherGuy/firefox-csshacks/tree/master/chrome/tab_close_button_always_on_hover.css made available under Mozilla Public License v. 2.0
See the above repository for updates as well as full license text. */

/* Always show tab close button on hover and never otherwise */
.tabbrowser-tab .tab-close-button{
  display:none;
}
.tabbrowser-tab:not([pinned]):hover .tab-close-button{
  display: flex !important;
  align-items: center;
}