gecko/toolkit/content/widgets/scrollbar.xml

<?xml version="1.0"?>

<bindings id="scrollbarBindings"
   xmlns="http://www.mozilla.org/xbl"
   xmlns:xul="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"
   xmlns:xbl="http://www.mozilla.org/xbl">
  
  <binding id="thumb" extends="xul:button">
    <content>
      <xul:gripper/>
    </content>
  </binding>

  <binding id="scrollbar-base">
    <handlers>
      <handler event="contextmenu" preventdefault="true" action="event.stopPropagation();"/>
      <handler event="click" preventdefault="true" action="event.stopPropagation();"/>
      <handler event="dblclick" action="event.stopPropagation();"/>
      <handler event="command" action="event.stopPropagation();"/>
    </handlers>
  </binding>

  <binding id="scrollbar" extends="chrome://global/content/bindings/scrollbar.xml#scrollbar-base">
    <content>
      <xul:scrollbarbutton sbattr="scrollbar-up-top" type="decrement" xbl:inherits="curpos,maxpos,disabled,sborient=orient"/>
      <xul:scrollbarbutton sbattr="scrollbar-down-top" type="increment" xbl:inherits="curpos,maxpos,disabled,sborient=orient"/>
      <xul:slider flex="1" xbl:inherits="disabled,curpos,maxpos,pageincrement,increment,orient,sborient=orient">
        <xul:thumb sbattr="scrollbar-thumb" xbl:inherits="orient,sborient=orient,collapsed=disabled" 
                   align="center" pack="center"/>
      </xul:slider>
      <xul:scrollbarbutton sbattr="scrollbar-up-bottom" type="decrement" xbl:inherits="curpos,maxpos,disabled,sborient=orient"/>
      <xul:scrollbarbutton sbattr="scrollbar-down-bottom" type="increment" xbl:inherits="curpos,maxpos,disabled,sborient=orient"/>
    </content>
  </binding>
</bindings>