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:

• Create a userChrome.css file
  ($FF_PROFILE_FOLDER/chrome/userChrome.css)
• Enter the code below (taken from here, which is basically the original of this article)

.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;
}