Rated 5 out of 5 stars
i've always had a hard time seeing the gray on gray scroll bar especially when the scroll bar is small on a long page, and loved this add-on to give full customization. what a shame to lose it with the release of quantum. hopefully in the future they will allow UI changes.
In favor of your eyes I suggest to move to Waterfox and keep using this add-on.
Mozilla has plans to add some kind of scrollbar customization support in the future, but nothing concrete yet.