Yank It! per Sarah

Replaces Firefox's autoscroll function on the middle mouse button with a middle-click-and-drag system that acts like a typical mobile browser with support for a flicking gesture to quickly scroll through a page. Flicking and inertia can be adjusted or disabled completely. This addon also has a built-in toggle in its popup menu so you can quickly enable or disable it and revert to default behavior without going to the addon settings page. Holding shift while middle-clicking will also force default Firefox behavior.