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Well, there´s a possibility that instead of being eaten, the hand was eaten away by an infection.
First off we need to know the species, since firebelly newt can refer to several. If they are Cynops orientalis, then agression is very rare and it might be an infection. If they are Pachytriton labiatus, then agression is a very fair possibility.
Also, try to upload a picture, it´´s usually rather easy to see if a wound has been caused by infection or agression.
Anyway, if the wound is clean and there is no fungal growth or further damage, then it might heal on it´s own and grow back in time.