Did not work for me, so I tried another one, "Yet Another Smooth Scrolling".
https://cup.aihedy.com/en-US/firefox/addon/yass-we

YASS at least works, but it really sucks compared to Smoothwheel. That was the best -- it was smoother than anything else, it had better controls, it worked on all web sites, and it worked with all mice (until Firefox Quantum broke compatibility.) Someone needs to resurrect this extension. I hate Firefox without it.

https://web.archive.org/web/20161123223319/https://cup.aihedy.com/en-US/firefox/addon/smoothwheel/

http://forums.mozillazine.org/viewtopic.php?f=38&t=3033273
http://smoothwheel.mozdev.org/