How do I enable pinch zoom in Firefox on a Mac OS X?

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/