Glad to hear that you like the add-on. I haven't had a chance to update the wiki page with information about why that new permission is required. I'm probably going to update that document later on today.

In short, this has to do with some adjustments that were made to make the add-on work properly for content that is embedded within other websites. Some older versions of Firefox will not share information about what websites are open unless they have that permission. Unfortunately, I can't apply the permission only to those versions of Firefox that need it, so I have to add that permission for every very of Firefox.

I plan on posting a more detailed explanation to the permissions page: https://github.com/WesleyBranton/Custom-Scrollbar/wiki/About-Permissions