Hi,
It's totally impossible to say anything from the info that you are giving, you have to provide info on how you are keeping him, how long you have had him, how long time has passed since its last meal, what temperature is it kept in, a description of what kind of firebellied newt it is or preferably a photo etc etc etc.
If it is a cynops orientalis(see photos on the site), it should be kept below 21C in a semiaquatic tank. How often do you clean the tank? Do you use filters? Have you checked the water quality?
Newts are easily stressed and often refuse to eat if stressed - in my experience cynops orientalis from pet shops are extremely sensitive and easily stressed. A newt survives a very long time without food - if it doesn't eat in a week it's nothing even though one should think about why....