Ok, open Firefox, and enter about:config in the address bar, then in the filter box, put browser.gesture, that will bring up all the touch gestures for Firefox. Once you have the touch settings showing, look for browser.gesture.pinch.in and set the value to cmd_fullZoomReduce, that should give you the zoom out, and to get the zoom in, look for browser.gesture.pinch.out and set that value to cmd_fullZoomEnlarge. Once you have those values set, there are 2 more settings you have to change. The next one you want to look for is browser.gesture.pinch.latched by default it's set to true you want to change that to false, then go to browser.gesture.pinch.threshold, which is set to 150 by default, you want to change that value to 10.

Report back whether it worked or not

I have changed the about:config settings, however, multi-touch still does not work in my browser. Firefox does recognize the tap and long-press gestures just not multi-touch gestures such pinch or a two-fingered swipe. Thus it seems that Firefox is having trouble detecting multiple points touching the screen simultaneously. I have tested a multi-touch demo with the Chromium browser and have verified it is not an issue with my system.

Furthermore, the default values were not as you suggested. the browser.gesture.pinch.latched was set to false by default and browser.gesture.pinch.threshold was set to 25 by default. I am not sure if this is of any interest, but I thought I should mention it.