Thin, dark scrollbars for Firefox 57+ Quantum. Designed to match the dark theme. Tested on Windows 10. Requires https://gist.github.com/Sporif/db6b3440fba0b1bcf5477afacf93f875

dark-scrollbar.as.css

/* remove incremental scroll buttons located at scrollbar ends.
   important note:
     only partially zero out height/width dimensions
     to prevent entire scrollbar from disappearing
     (ie, scrollbarbutton collapse results in scrollbar collapse)
*/
scrollbar[orient='vertical'] > scrollbarbutton
{
  min-height: 0mm !important;
}

scrollbar[orient='horizontal'] > scrollbarbutton
{
  min-width: 0mm !important;
}

/* boilerplate */
scrollbar,
scrollbar > slider,
scrollbar > slider > thumb,
scrollbar > scrollbarbutton
{
/*   min-width: 2px important;
  min-height: /*[[width]]*/;
}

scrollbar > slider,
scrollbar > slider > thumb,
scrollbar > scrollbarbutton
{
  -moz-appearance: none !important;
}

scrollbar, scrollbar > scrollbarbutton
{
  border: none !important;
  background: #111!important;
  width: 10px !important;
  min-width: 10px !important;
}

scrollbar[orient='horizontal'] > slider > thumb
{
/*   min-width: [[thumb_min_length]] */;
}

scrollbar[orient='vertical'] > slider > thumb
{
 /* min-height: /*[[thumb_min_length]]*/;
} 


/* scrollbar style */

scrollbar
{
  border: none !important;
  background: #333!important;
}

scrollbar > slider
{
  border: none !important;
  background: #333!important;
}

scrollbar > slider > thumb
{
  border: none !important;
  background: #555!important;
  border: 2px solid;
  border-radius: 0px;
  /*border-radius: /*[[thumb_corner_radius]]*/;
}

scrollbar > slider > track
{
  background: #111!important;
}

scrollbar > slider > track:active
{
  background: #111!important;
}

scrollbar > slider > thumb:focus,
scrollbar > slider > thumb:hover,
scrollbar > slider > thumb:active
{
  background-color: #308FE2!important;
}