gecko-dev/toolkit/themes/pinstripe/global/dropmarker.css

dropmarker {
  -moz-appearance: menulist-button;
  width: 16px;
  -moz-box-align: center;
  -moz-box-pack: center;
  border: 2px solid;
  -moz-border-top-colors: ThreeDLightShadow ThreeDHighlight;
  -moz-border-right-colors: ThreeDDarkShadow ThreeDShadow;
  -moz-border-bottom-colors: ThreeDDarkShadow ThreeDShadow;
  -moz-border-left-colors: ThreeDLightShadow ThreeDHighlight;
  background-color: -moz-Dialog;
  padding: 1px;
  list-style-image: url("chrome://global/skin/arrow/arrow-dn.gif");
  -moz-image-region: auto;
}

dropmarker:hover:active:not([disabled="true"]) {
  -moz-border-top-colors: ThreeDShadow ThreeDFace;
  -moz-border-right-colors: ThreeDShadow ThreeDFace;
  -moz-border-bottom-colors: ThreeDShadow ThreeDFace;
  -moz-border-left-colors: ThreeDShadow ThreeDFace;
  padding: 2px 0 0 2px;
}

dropmarker[disabled="true"] {
  list-style-image: url("chrome://global/skin/arrow/arrow-dn-dis.gif");
}