2: The Borg can adapt to fire. That's ridiculous. If the Borg can't adapt to a holographic Tommy gun (Star Trek: First Contact), then they can't adapt to fire.
There is a difference between Plasma and Kinetic Weaponry. But I think a better argument would be to mention how the Borg can survive the attacks from the flamethrower, but die instantly when they fall into the Plasma Floor in Infected.
But its obvious if the Borg weren't able to adapt to this weapon, then everyone would use Flamethrowers against the Borg.