Rated 4 out of 5 stars
The Noia themes are all absolutely hideous but the transparent theme looks good. I wish there was a preview (or an interactive/live mockup if a preview isn't possible) so that you know what was happening when configuring your bars.This review is for a previous version of the add-on (1.2.4).
Firefox only renders/displays/updates scrollbars after a page gets (re)loaded, so there can not be a live preview of your changes.
This add-ons size is less than 90KB. Adding any live mockups / preredered images of countless possible scrollbar variations would not only increase the add-ons size by an unknown factor, but would also require a large amount of additional code without any benefits after configuration.
If you really need a live preview, you can open your library window (bookmarks, history... etc.) and a folder that offers a scrollbar within the library. Most changes inside NewScrollbars add-on go live on that window, after you change something.