kind of a late response but:
Fire bellies are very hardy frogs and will tolerate many different kind of conditions. So indeed, when humidity requirements are met, they will survive in a tank as you describe.
HOWEVER; they will not be particularly happy as they are highly aquatic for a toad (they're also not real toads actually).
You will notice this in their behaviour.
They will probably be less active because they will never experience a spring/summer feeling.
At least in my experience frogs that had acces to a large water area were much more fun to watch.