I actually was looking for information on how to enable this feature as I had thought it had become disabled with the newer Firefox releases. For those who are not familiar with autoscrolling, it allows you to quickly scroll through a page by clicking the mouse scroll wheel and then moving the mouse up or down. When autoscroll is activated, the mouse cursor changes to this:

For those who use their middle button for other uses and can not control the behavior via their mouse software (more on this shortly), the autoscrolling can be chnaged in most all versions of Firefox via:

 
• Windows: Tools >> Options… >> Advanced >> in the Browsing section,  Use autoscrolling
• Linux: Edit >> Preferences >> Advanced >> in the Browsing section,  Use autoscrolling
• Mac: Firefox >> Preferences… >> Advanced >> in the Browsing section,  Use autoscrolling

 

 

After some more searching (which lead to me finding some for mouse-wheel related tweaks), I found that my mouse software had the mouse-wheel click set for zoom.

Note: On Windows this is normal turned on by default, whereas on Linux this is turned off.