Firefox Middle Mouse Button Scroll
Edit ➜ Preferences ➜ Advanced ➜ Check 'Use autoscrolling'
Edit 2020-11-23: this answer is verified with Firefore 62.0.3
Edit 2018-03-14: this answer is verified with Firefore 58.02
Just to precise, since version 57 of Firefox the path is the following: Menu ➜ Preferences ➜ General, then all the way down in the Browsing section check the box Use auto scrolling.
You can also just type auto in the search field once you're in the Preferences window.
See this screenshot:
