The true Istari

[Eönwë]

The staff is fo channeling the power inside the Istari. While clothed in the bodis of men they could could not use their full Maia powers. I think that they were only allowed to use a certain namount of their magic when they left Valinor, and also that Saruman got the most power, then Gandalf, then Radagast, then Alatar and Pallando, because the Valar allowed the leader to use more power.That is why, when Gandalf gets a new staff, his powers increase, because it allows him to reach more of his real power. So when he is reborn, he takes Saruman's place, and gets more power (though nothing near his full potential).

By the way, Gandalf did kill quite a few things with fire (Actually, mostly Wolves) in LOTR and The Hobbit.