I believe this is an issue with Firefox itself, as it does it on various OS's, and on various hardware. I know how to get it work on a MacBook multi-touch touchpad running Ubuntu, so I'll show you how to do it there, and you should be able to get it to work on your touch screen as well, as it's a setting within Firefox, not the system.
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
That is how I got all the multi-touch features working for me, I hope it works for you as well, good luck.