The Adjust Scrollbar Size setting controls the width of the scrollbar on pages where the text is long enough to require scrolling.
Originally, this setting didn’t work in either Firefox or Chrome. When I retested recently, I discovered that the setting now works in Chrome but still doesn’t work in Firefox.
Below is a short animated GIF file showing the better scrollbar options in action. I followed the same order as shown in the Toolkit:
Disclaimer: I’m a Mac user. All of my videos and screenshots are taken on my MacBook.
If you really don’t like scrollbars and you only use Chrome, this setting will probably make you happy. Me, I struggle to see the point.