Rated 5 out of 5 stars
This is an excellent add-on Aris well done.
The only thing i would like to see this add-on include would be a Transparency slider and this would be absolutely perfect.
Level of trancparency can be set in Windows settings or do you mean with "slider" scrollbars? If so there is my extension called "NewScrollbars" ( https://addons.mozilla.org/addon/noiascrollbars/versions/ ) where you can set a semitransparent default scrollbar in version 1.0.3pre4+.