GlassMyFox is Best I've seen!! Rated 5 out of 5 stars

I have reviewed this once but just have to tell ya, I love this!!

I would also like to suggest transparent scroll bar. I saw a reply to a post but there are no setting that I can find to set transparency level on the scroll bar.

This review is for a previous version of the add-on (1.0.7.1-signed).  This user has other reviews of this add-on.

It is not possible to modify main window scrollbars from a bootstrapped (restartless) extension like GMF. But
with "NewScrollbars" ( https://addons.mozilla.org/addon/noiascrollbars/versions/ ) you can set semitransparent default scrollbars (v1.0.3pre4+).