First I got the problem solved that 2-finger-gesture scrolling did not work at all: https://support.mozilla.org/en-US/questions/895816. Thanks for that! With "smooth scrolling" turned on in the options, touchpad gesture scrolling is smooth in FF9 but only in 5-line-steps. No stopping in between possible. If touchpad scrolling is performed in IE9 once, scrolling in FF9 is completely smooth afterwards. If the gesture is perfomed once in Google Chrome 16 scrolling in FF9 is turned back to 5-step-scrolling. Chrome 16 and IE9 can be used to turn off and on the completely smooth scrolling in FF9!