This is a primary extension (choose Use Custom Colors) which I think should be a default part of FF.

And changing widget.non-native-theme.scrollbar.style to 4 (from 0) in about:config (and hit the Save checkmark) makes it wider.

And I read with disappointment about the response to the developer by FF to enable wider widths:

Rather, as Custom Scrollbars beatifically examples, customization enhances browsers. Thank God.

(Note that Custom Scrollbars will not work in Firefox system pages as this).