In about:config

  widget.non-native-theme.scrollbar.size.override 20 
  widget.non-native-theme.scrollbar.style  4
  widget.gtk.overlay-scrollbars.enabled   false

I even use a custom gtk.css to improve things further:

  scrollbar, scrollbar button, scrollbar slider {
    -GtkScrollbar-has-backward-stepper: true;
    -GtkScrollbar-has-secondary-forward-stepper: true;
    -GtkScrollbar-has-forward-stepper: true;
    min-width: 20px;
    min-height: 20px;
    border-radius: 0;
  }

It's a shame we have to resort to this to get decent scrollbar behavior.