Nice add-on. I would love to be able to choose the width in pixels instead of Default/thin options (and maybe one day an "Hide when not is use" option :D).
I'd love to be able to do this. However, it's not currently possible due to the approach that Firefox takes when it comes to customizing the scrollbars. I really hope it will become possible in the future though because I think it would be really cool to give users like yourself more control over the appearance of the browser.
PS I left you a more detailed explanation on the request you posted on the GitHub repository: https://github.com/WesleyBranton/Custom-Scrollbar/issues/14
