In Firefox, I also add the following to userChrome.css to make the scroll thumb high-contrast versus the scroll bar itself. The default Firefox colors are, in my opinion, too low-contrast.

    :root {
      scrollbar-color: rgb(35,75,240) rgb(220,222,224) !important;
    }

The first color is for the thumb (the part you pick up and drag) and the second color is for the remainder of the scrollbar.

Resulting appearance: https://imgur.com/a/A1LTS4M