put in user.js
user_pref("apz.overscroll.enabled", true); // DEFAULT NON-LINUX
user_pref("general.smoothScroll", true); // DEFAULT
user_pref("general.smoothScroll.msdPhysics.continuousMotionMaxDeltaMS", 12);
user_pref("general.smoothScroll.msdPhysics.enabled", true);
user_pref("general.smoothScroll.msdPhysics.motionBeginSpringConstant", 600);
user_pref("general.smoothScroll.msdPhysics.regularSpringConstant", 650);
user_pref("general.smoothScroll.msdPhysics.slowdownMinDeltaMS", 25);
user_pref("general.smoothScroll.msdPhysics.slowdownMinDeltaRatio", 2.0);
user_pref("general.smoothScroll.msdPhysics.slowdownSpringConstant", 250);
user_pref("general.smoothScroll.currentVelocityWeighting", 1.0);
user_pref("general.smoothScroll.stopDecelerationWeighting", 1.0);
user_pref("mousewheel.default.delta_multiplier_y", 275); // 250-400; adjust this number to your liking