It stopped working for me after a Firefox update but I did get it to work again after fiddling around toggling settings in about:config and uninstalling/reinstalling the extension a few times. Not fun thanks to the FF update but at least it is working again.

I'm not sure of exactly what did it but if anyone else is having issues, try uninstalling YASS, enable FF's default smooth scroll, reset the general.smoothScroll.mouseWheel.migrationPercent setting to 100, and toggle mousewheel.enable_pixel_scrolling off then back on again. Leave both pixel scrolling and FF's smooth scroll enabled, restart FF, and reinstall YASS. Might work but YMMV