If the NYFD had them would the twin towers have fallen?
An interesting question. I haven't heard fire chiefs or 9/11 investigators address this specific scenario. I also haven't heard of fire chiefs in large cities clamoring for a fleet of airborne water cannons to fight high-rise fires either before or since 9/11. This could, of course, possibly be due entirely to the monumental, breathtaking cost of helicopter water cannons coupled with the comparatively few high-rise fires that they'd be called to fight.
I'd think that a computer-modeled scenario, implementing an airborne water cannon firefighting attack, might be possible to create which might shed some light on the matter, although the complexity of programming such a scenario sounds utterly monumental. I'm thinking that the difficulty of accurately programming all of the variables would make any model's results suspect and inconclusive at best, but I'm not an expert on supercomputer-driven scenario-modeling.
My initial guess would be that they wouldn't have helped much, due to the tremendous volume of fuel present and how far inside the building, away from the walls, that much of the fuel would have been. Even the powerful water cannons on one of these helicopters would probably not have been able to deliver enough water or foam at a high enough rate at enough of a depth into the building to be very effective, but who knows, it might have slowed the fire down enough to allow more people to evacuate before the actual collapse.
At this point in time, I believe only God knows the complete answer to your question ;-)