browser.gesture.pinch.in             cmd_fullZoomReduce
 browser.gesture.pinch.in.shift       cmd_fullZoomReset
 browser.gesture.pinch.out            cmd_fullZoomEnlarge
 browser.gesture.pinch.out.shift      cmd_fullZoomReset

I am not sure is it work will not . I just try to give a solution based on your problem. *Open a new tab. *Type in about:config and press Enter. *In the search pane, type pinch and press Enter. *Enter these values for the pinch.in and pinch.out prefs: browser.gesture.pinch.in cmd_fullZoomReduce browser.gesture.pinch.in.shift cmd_fullZoomReset browser.gesture.pinch.out cmd_fullZoomEnlarge browser.gesture.pinch.out.shift cmd_fullZoomReset

You can look at this extension: *pinchy: https://addons.mozilla.org/firefox/addon/pinchy/