IME, firebelly newts are very hardy. I have had one since 1992, and he's been through a lot and just won't die! He got out of the kritter keeper he came in (my aunt gave it to me for my 8th birthday) and got out one time was went missing for at least 3 days! 8O He showed up in the middle of the bathroom floor, dry as can be, but we put him back in and all was ok. over the years, he got upgraded to a terrarium where he still lives. Not that I recommend it, but we never did water changes on him, only added aged water (which, as I now know, most likely has chloramines in it!). When we fed him, he got bloodworms and occasionally he'd eat or at least chew on the tails of goldfish we'd but in there. There were a few years in there where he'd get fed maybe 1 a month, if he was lucky. He was in the house so temps changed a bit with the seasons (not that it's very much, seeing as I live in Sounthern Cali) and he never has had a heater.
I DO NOT RECOMMEND THIS TREATMENT OF NEWTS, but I do wonder, as the life expectany is rather low, especially in captivity, what kept him and is still keeping him alive... When I went with my brother to a reptile show about 5 years ago, when I would have had the newt for 9 years, the people at a booth with newts were shocked, and the books they had listed the age to be 5-7 years as max!