RichScrollbar extension, RSB for friends, is quite simple and not intrusive but technically is a big compromise.
It is written as XBL component and runs inside chrome context and browser content context.
Under Firefox 3 beta1 and beta2 RSB after some fix worked like a charm, when beta3 arrived RSB was broken.
Mark Finkle helped me to understand why RSB didn’t work and we discover FF3 fixed a security bug on XBL.
Now it is impossible to make RSB functional on browser windows, it works only on chrome dialogs.
I surrend, period! I have not enough technical knowledge to dig for a workaround.
I consider terminated the RSB development, the XBL documentation is poor and all my help requests haven’t received reply except Mark.
Honestly I’m sad, I love RSB and I prefer it to Mouse Gestures, I’ll continue to search for a fix but it is very difficult to find.
Mark helped me and I would say thank to him, great people work at Mozilla.