I have one of those Microsoft mice here at work and its wheel is quite
light to the touch. It is so light in fact, that I often click it while
scrolling. By default, clicking the scroll wheel in Firefox puts it
into scrolling mode, showing this icon:
Now when you move the mouse up and down, the page will scroll.
That is so annoying!
It took me over 20 minutes of Googling to find how to disable that feature, so I’m documenting it here.
The option that controls it in Firefox is Tools > Options > Advanced > General > Browsing > Use autoscrolling.
Apparently “auto scrolling” is the name of this obscure feature, which is now forever disabled on all my machines.