I came across this issue on multiple of my machines. usually reading webpage which is fairly long so have to scroll multiple times to get to bottom. I would scroll down and stop where I want it to stop, when it gets there, it will move either a bit up, or a bit down by itself and that frustrates me.
I think problem resides in firefox and not other browser but I could be wrong.
Ive found a solution which may or may not work but so far it's been pretty good. start -> search -> type in mouse. go to pointer options tab and uncheck the box which says "enhance pointer precision" and seems to do the trick.