Just to second what Par is saying, if he saw a fireball then it wasn't a steel-cutting explosive. Might have been a variant of a cratering charge, but that brings in another problem. A fireball like you see in movie explosions is mostly gasoline, and would not have been effective in initiating a collapse.
If a conventional explsoive, as those used for sutting steel and metal, there would not have been a fireball such as that. A blast wave, yes. Dust cloud and shrapnel, sure. Little heat and flame, however, at least away from the immediate area of the detonation.
A cratering charge is a slower-moving explosive, and they'll produce a bit more of a fireball usually. However, they don't do very well at cutting through steel or other hard surfaces, so they wouldn't be effective. You'd need a heck of a lot more of them to do significant damage, which would have been noticeable by a lot more people than a single janitor in the basement.
And a gasoline fire, of course, would have been a fart in the wind.
There's another side to this, as well. Almost everyone in the building felt/heard the plane impact, from the top to the bottem of the building. Yet, the troothers say this is not enough damage to collapse the building. However, apparently so little energy was released formt he explosives that the only on who had any idea they were there was a janitor in the basement, indicating that they were several orders of magnitude less powerful than the jet impact. Yet, somehow, this does initiate a collapse.
It's these contradictory applications of logic that bring ridicule, not the idea of CT itself. IF there were credible evidence, it might be different. As it is, the entiore CT argument is speculation based on argument from ignorance, circular reasoning, and similar fallacies.