If they had him in with lizards and tree frogs, I doubt they were feeding him the correct foods, or that he even got enough. MBD sounds like it could be a definite possibility like Lija said. Another sign of it is that it might look like it's having trouble holding itself up on its front legs.

Only other thing; I'm sure you did, but make sure to deep clean that plant. Firebellies produce some irritating toxins on their skin even for humans, so probably would have a worse effect on another frog.