The scrollbars in firefox don't look great on dark themed websites, which I strive to do for most websites I visit, additionally the scrollbar popping around according to where your mouse is is incredibly jarring. I understand that the new web extensions don't provide a way to do this anymore (there were numerous addons pre-57). The only solution right now is to change the size of the scrollbars in windows with a registry tweak, or stretch the scrollbar off the screen with a userChrome.css tweak. Neither of those are very good options. If WebExtensions aren't going to have the ability to make this change, I know a lot of us would appreciate it being an option in preferences.