@dietrich's userChrome.css file for Firefox

userChrome.css

/*
* Do not remove the @namespace line -- it's required for correct functioning
*/
@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"); /* set default namespace to XUL */
 
/*
Disable tab throbber to stop battery death.

https://bugzilla.mozilla.org/show_bug.cgi?id=1422090#c67
*/
.tab-throbber { visibility: hidden !important; }

/*
Remove the “close other tabs” context menu option.
It is destructive, and there is no undo.
*/
#context_closeOtherTabs {
  display: none !important;
}

/*
Everything below here is to enable the tab close button to be
visible when you hover over the favicon on inactive tabs.
*/

/* move favicon, throbber and text to the right so that the close button appears on the left */
.tabbrowser-tab .tab-icon-image {
  -moz-box-ordinal-group: 2 !important;
}
 
.tabbrowser-tab .tab-throbber {
  -moz-box-ordinal-group: 2 !important;
}
 
.tabbrowser-tab .tab-label {
  -moz-box-ordinal-group: 3 !important;
}
 
/* hide close button initially */
.tabbrowser-tab .tab-close-button {
  display: none !important;
}
 
/* show close + hide throbber & favicon on hover */
.tabbrowser-tab:hover .tab-close-button {
  display: block !important;
}
 
.tabbrowser-tab:hover .tab-icon-image {
  display: none !important;
}
 
.tabbrowser-tab:hover .tab-throbber {
  display: none !important;
}
 
/* app tabs should not behave like regular tabs, so let them stay normal (viz. don't show close on hover) */
.tabbrowser-tab[pinned]:hover .tab-close-button {
  display: none !important;
}
 
.tabbrowser-tab[pinned]:hover .tab-icon-image {
  display: block !important;
}