Tap on the first box and then quickly on the second one. On Firefox for Android 68.1.1, the counter will only increment once.

It seems the 2nd event is not fired at all if its touchend happens too soon after the 1st. "touchstart/touchend" on the other hand doesn't seem to have the problem. If you tap them in quick succession starting with the 1st, all events from the top one will be caught and all from the bottom will be ignored.