Just installed it and I like the look of it but haven’t tested changing the colors yet.

      …Firefox users who have installed a new theme in the browser may also notice that the initial scrollbar design might not be that fitting for the selected theme.  The New Scrollbars add-on for the Firefox web browser offers to replace the default scrollbar with another one. It ships with a total of seven scrollbar colors that are made available for themes with light and dark backgrounds, bringing the overall total of available custom scrollbars to 14… “

Source: Ghacks
 Change The Firefox Scrollbar