Scroll fast or precise through long documents with pressed middle mouse button.
Holding down Alt or Ctrl key while dragging with middle mouse button, or double-clicking middle button, enables you to use the browser's implementation of middle-click scroll.
Holding down Shift key while dragging with middle mouse button enables you to scroll slowly and precisely.
Note: For the time being the extension scrolls only the whole page and not different parts of it. If you are unable to scroll given page that's the reason why.
Also, for the time being the hotkeys are non configurable. The current implementation of Firefox for configurable hotkeys does not permit single modifier key (Ctrl, Alt, Shift, Meta etc.) as hotkey.