Thanks for providing a "better than nothing" solution, but brace yourself, constructive criticism incoming ;)

- by default, we must keep the middle button pressed and move our mouse down/up, a lot, following which, after a delay, there is a brutal movement start.
Keywords: move a lot, delay, brutal.

- the "natural" movement one would expect would be that once the middle mouse button is pressed, the slightest movement up/down causes the slightest scrolling, while a stronger delta (my own non-techie term, the distance between "imaginary" pointer position and position of the pointer when we pressed the middle mouse button) will cause a faster scrolling
In a nutshell, there ought to be, both, no delay before the scrolling begins, and, imperatively (for it to feel smooth) a variation in the scrolling speed

- Lastly, we shouldn't have to keep the middle button pressed. More natural behaviour would be: we press it once. The plugin observes how much we move vertically from there and maintains a variable scrolling speed, maintained until we release it with a left.right.middle click.

I know, it's soooo easy to ask for all of that when you can't code an hello world. But, well, that's what was available before with XUL extensions, it's an ultimate goal, thanks already ;)