Firefox 64 does support customizing the scrollbars and you can e.g. see them being used in the dev tools of Firefox itself:
However, as you see, on Linux they are mostly just rectangles on top of each other, which is not really a great design/user experience. The MDN docs example also shows this:
This is not really visually appealing. It would be great to have a little better support. In GNOME you e.g. have native styles for dark and light scrollbars, these could be used if only dark or light is specified by the developer.