nsIDOMSimpleGestureEvent
mozmagnifygestureupdate - generated periodically while the user is continuing the magnify ("pinch") gesture.
... the "delta" value represents the movement since the last mozmagnifygesturestart or mozmagnifygestureupdate event.
...if you only want to receive a single event when the magnify gesture is complete, you only need to hook this event and can safely ignore the mozmagnifygesturestart and the mozmagnifygestureupdate events.

Mouse gesture events - Developer guides
mozmagnifygestureupdate the mozmagnifygestureupdate event is sent periodically while processing a magnify gesture, to provide updated status information.
... the event's delta value represents the amount by which the gesture has moved since the mozmagnifygesturestart or mozmagnifygestureupdate event.
...if you only care about the end results of the pinch gesture, you can simply watch for this event; however, if you want to provide feedback during the handling of the gesture, you should also watch the mozmagnifygesturestart and mozmagnifygestureupdate events.

Event reference
mozmagnifygestureupdate addons specific two touch points move away from each other (after a mozmagnifygesturestart).
... mozmagnifygesture addons specific two touch points moved away from each other (after a sequence of mozmagnifygestureupdate).

nsIDOMWindowUtils
the event types supported are: mozswipegesturemaystart, mozswipegesturestart, mozswipegestureupdate, mozswipegestureend, mozswipegesture, mozmagnifygesturestart, mozmagnifygestureupdate, mozmagnifygesture, mozrotategesturestart, mozrotategestureupdate, mozrotategesture, mozpresstapgesture, moztapgesture, and mozedgeuigesture.