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.

Thanks for the feedback! Sorry to hear you are having issues. I'd love to take a look into the Magic Mouse issue for you, although it may be difficult since I don't have a Mac.

If you could either file a bug report on GitHub (https://github.com/WesleyBranton/Custom-Scrollbar/issues/new) or email me at addons@wesleybranton.com with more information, it would be much appreciated. It's helpful if you include your current settings and, if the issue is with a specific website, the URL for that website. Thanks!