Firefox 4 disables two finger pinch/zoom gestures on a Mac. Here is how to make pinch/zoom work again.
To enable pinch/zoom in Firefox:
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
Mac OS X Hints
http://hints.macworld.com/article.php?story=20110322102812402