nsIDOMSimpleGestureEvent
moztapgesture - generated when the user executes a two finger tap gesture on the input device.

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

Event reference
moztapgesture addons specific two touch points are tapped on the touch surface.