Ankyra Sound [VS] I tried to kill the pain, I bought some wine and hopped a train

Firefly was reacting to her challenge with some care as he did not intend to harm a child, let alone the child of someone like Caiaphas whom he admired so fiercely. He thought it was over. That she would slink off to nurse her emotional wounds, maybe, and everything would be concluded. Except that wasn't what happened — rather, she reacted to his teeth at her throat and appeared to dissolve in to a puddle of fright, uncontrollable and feral. 

He reacted to that without thinking because he was so shocked by the abrupt shift of her behavior - his tail arched across his back and he, without thinking or meaning to, removed his teeth from her throat but roared right back at her, devolving almost instantaneously to a creature intent on forcing the issue of his dominance. He met her growling and her fear with his determination, and loomed over her with a grimace, using his body to get his point across.