After upgrading to Firefox 59.0 (on Windows 7 x64 with the Classic theme), I noticed the right-click menu within the browser had a new flat look, similar to the one in Chrome.
I don't like Chrome and I don't like a flat context menu in my browser. I hate change So here is a little piece of code for your userChrome.css to bring back the old 3D look:
border: 2px solid !important;
-moz-border-top-colors: ThreeDLightShadow ThreeDHighlight !important;
-moz-border-right-colors: ThreeDDarkShadow ThreeDShadow !important;
-moz-border-bottom-colors: ThreeDDarkShadow ThreeDShadow !important;
-moz-border-left-colors: ThreeDLightShadow ThreeDHighlight !important;
padding: 1px !important;
Enjoy the result!