If you want the menus to appear native to the Windows Classic appearance, add the following code to your userChrome.css file:

/* Windows Classic (9x/Me/2000) style Menus */
menupopup, popup {
   border: 2px solid !important;
   -moz-border-top-colors: ThreeDLightShadow ThreeDHighlight;
   -moz-border-right-colors: ThreeDDarkShadow ThreeDShadow;
   -moz-border-bottom-colors: ThreeDDarkShadow ThreeDShadow;
   -moz-border-left-colors: ThreeDLightShadow ThreeDHighlight;
   padding: 1px !important;
}
menubar > menu[disabled=”true”] {
   border: 1px solid transparent !important;
}
menubar > menu[_moz-menuactive=”true”] {
   border-top: 1px solid ThreeDHighlight !important;
   border-right: 1px solid ThreeDShadow !important;
   border-bottom: 1px solid ThreeDShadow !important;
   border-left: 1px solid ThreeDHighlight !important;
   background-color: transparent !important;
   color: MenuText !important;
}
menubar > menu[_moz-menuactive=”true”][open=”true”]  {
   border-top: 1px solid ThreeDShadow !important;
   border-right: 1px solid ThreeDHighlight !important;
   border-bottom: 1px solid ThreeDHighlight !important;
   border-left: 1px solid ThreeDShadow !important;
}
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