Its not that they bite hard. They have spike-like projections coming out of their mouth, at the frog that act like teeth, to help them restrain struggling prey. You get bit...you will bleed lol.

As for handling firebellys. Any amphibian is like that. They all have really sensitive, thin skin. Dry hands, while holding them, can hurt then, or tear their skin. Any chemicals you have on your hands, they could absorb into their body's, causing damage. Its generally not a good practice to handle amphibians. If you do, wet your hands down, with water first.