Middle mouse auto scroll.

I should add that the chrome one is not as good because you have to click then let go of the click and then drag vs. the firefox way of click and hold and drag or click and let go and drag. it's a lot easier to use.

No, such a plugin system does not exist. It sounds like some kind of system-hack would also be more appropriate although I'm not sure I understand exactly what you are asking for. Is it equivalent to the two-finger track-pad scroll on OS X, but instead using the middle button of a three-button mouse?

it's similar to two finger scroll but automatic if that makes sense. you press the middle mouse button(or an emulation of it via some 3rd party software like magicPrefs see: http://i.imgur.com/U0sJylh.png) then you either release the middle mouse button or hold it down. This sets a reference point on the screen that acts as the baseline y value. then depending on how far above or below the reference point you are the window scrolls down at a constant rate relative to that distance. The farther you are from the baseline the faster it scrolls. Try it out in firefox to see what I mean.

I recorded a screen capture of it in action:

https://www.youtube.com/watch?v=CdByD-I2pa4

When you hold down the middle mouse button it does the scroll until you release the middle mouse button again. When you click the middle mouse button it does it until you click any other button.

As Mozilla uses it in their code and their code is under a pretty permissive license you can look at it here:

https://dxr.mozilla.org/mozilla-central/rev/63ca686c3f1e870649b6d9c...

It seems like a fairly easy thing to inject into the webview using javascript.