Glad to hear that you like the add-on. I've tested that setting on Windows and everything seems to be working properly. It could possibly be a platform-specific issue. If you could email me your operating system and Firefox version to addons@wesleybranton.com, I'd be happy to try and look into that issue.

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

Unfortunately, Firefox currently has very limited support for adjusting scrollbar widths. I'd love to expand the options available for that when more options are added to Firefox.

As for customizing this page, that's a restriction Firefox has for all add-ons. There are some protected pages (such as internal browser settings pages or Mozilla-owned pages), which add-ons are not allowed to edit. The intention is to prevent add-ons from injecting false or dangerous information on the pages, but it also means that the Custom Scrollbars add-on can't modify the scrollbars on this page.