On Firefox 57.0, it appears that TouchHandler's _handleTouchEnd does not fire on the first touch event. Perhaps because the touch begins before the touchend event listener is added. This results in the second _handleTouchStart not being fully followed (because listeners are already added). The second and all even touches then properly fire _handleTouchEnd. The third and all odd touches also do not fire the _handleTouchEnd method.

Roughly, this appears to a user as dropping every other swipe event.