So, you’re clicking around, trying to find the end of the Internet, when you accidentally click your middle mouse button and this pops up:
This means AutoScroll has been enabled and all you have to do now is move your mouse around to browse the page. I’ve never been a big fan of it, though, so I went hunting around for how to turn it off.
In Firefox, go to Tools>Options. Make sure the Advanced icon is selected and that you’re on the General Tab.
Under the Browsing header, un-check Use autoscrolling. That’s all there is to it!
If you ever want to turn it back on again, just follow the steps above and re-check Use autoscrolling.