首楞嚴義疏注經 5.352

5.352

因觀火性，得真三昧，以火為入道初門，故云火頭。火能破壞一切諸法，故發大願，為力士身破魔護法也。

Due to contemplating the nature of fire, he obtained true samādhi. Since fire was the initial gateway for entering the Way, he is called 'Fire-Head'. Fire can destroy all dharmas; hence he made a great vow to take the form of a mighty warrior to defeat māras and protect the Dharma.