Scroll-Anywhere

Allows you to scroll the webpage no matter where the pointer is, as if the pointer was currently over the scrollbar.

Simply hold down the middle button anywhere on the webpage and move the mouse in any direction.

You can also go into the extension's settings page and change the direction of the scroll so that it behaves like touch devices.

As an optimization, if you hold down the middle button, move the mouse a little and then very quickly release the button, you can continue to scroll the webpage without needing to keep the mouse button pressed.

This extension is open source: https://github.com/em-te/webextension-scrollbar-anywhere

Caveat: This extension cannot work on the Extensions Gallery (this website).