Add documentation for Firefox configuration settings

[WesleyBranton]

There are two requests that consistently appear in the add-on feedback for the Firefox version:

• Make scrollbars wider
• Make buttons visible (Mac/Linux users)

Unfortunately, Firefox does not currently allow the add-on to make these modifications, unlike the Chromium-based browsers. Documentation needs to be added to the wiki about the scrollbar theme configuration settings that Firefox offers, mainly widget.non-native-theme.scrollbar.style. A directly link to the wiki page should be added to add-on settings page to help users find out how to make these modifications.

[WesleyBranton]

I've updated the priority of this issue because I recently updated to Windows 11 and I'm shocked to see that the custom width option does not seem to work at all in Firefox. In fact, the Windows 11 scrollbars work quite differently from other operating systems, so additional work may need to be done on the add-on.