The problem has never been with Firefox restricting access to settings. The problem is that Firefox has implemented scrollbars using the W3C standard, which doesn't support a wide scrollbar. This means that the browser simply does not support any width other than default, thin, and none. You can see the specification here: https://drafts.csswg.org/css-scrollbars/#scrollbar-width

You can, as you pointed out, change the scrollbar width globally across Firefox using some modifications to about:config. The challenge with this is that the user needs to do it manually since the add-on cannot change any settings in about:config. Long ago, add-ons used to be able change settings there, but when Firefox converted everything to use webextension technology that ability was removed and will not come back: https://discourse.mozilla.org/t/webextension-read-write-access-to-about-config/12268

The Toggle Resist Fingerprinting add-on that you referenced does have access to a very limited set of privacy-related settings, but only because Firefox has chosen to make those settings available to add-ons: https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/privacy