nsIDOMSimpleGestureEvent
(xxx not implemented on mac) mozpresstapgesture - generated when the user executes a press and tap two finger gesture (first finger down, second finger down, second finger up, first finger up) 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
mozpresstapgesture addons specific a "press-tap" gesture happened on the touch surface (first finger down, second finger down, second finger up, first finger up).