Works nicely, with a few exceptions Rated 4 out of 5 stars

Really nice addon that lets you style your scrollbar.
I went for a simple, low contrast (high on hover) and compact scrollbar for future browsing.

However, there is still a lot of room for improvement and fixing:
Highlighting on hover works, however not while one drags it. Also, unlike the regular scrollbar, highlighting only starts once you are over the dragable slider or button(s). With CSS3 animation, maybe some fluent transition between hover/active/idle would be possible as well.
It also unfortunately breaks the responsive design scrollbars, which are different from the usual scrollbars. To enter the responsive design mode, press CTRL + Shift + M.
"Scrollbars on top of webcontent", being a nice new feature, does not always work. E.g. It fails in the Typo3 CMS backend, as the scrollbar is nested below certain elements...
"Transparent scrollbar background" only works properly with "Scrollbars on top of webcontent" enabled. Maybe this should be a requirement for checking the box?
A color transparency setting would also be nice (eg. #ffaabbcc), especially when used with "Scrollbars on top of webcontent".

Keep up the good work!

This review is for a previous version of the add-on (1.2.1.1-signed). 

Thanks for the review and your suggestions.

You are right, there is always room for improvement. ;-)

Scrollbars in Firefox are always very sensitive and don't work correctly for all cases. Some features especially the one with transparency you are referring to works best on Windows AeroGlass.