Firefox close tab buttons on mouse hover

I used to have Firefox’s TabMix Plus addon installed. And the feature I missed most – surprisingly – is to have the “close tab” buttons appear on a tab when you hover the mouse over it.

Googling a little bit told me how to bring it back:

.tabbrowser-tab:not([selected]):not([pinned]) .tab-close-button {
  visibility: hidden !important;
  margin-left: -16px !important;
}
.tabbrowser-tab:not([selected]):not([pinned]):hover .tab-close-button { visibility: visible !important; margin-left: 0px !important; display: -moz-box !important; }