Actually I do not think that fire-bellies are one of the species of caudates that can easily regenerate limbs. If you do not want a pet with a missing leg, you should not have bought it in the first place.
I am not exactly clear on what your fbns condition is, I couldn't tell from your post, but it sounds like it could be a broken leg? In that case you should take it to a vet, if it is not already irreperable by now. You could also attempt to fix it yourself, but since I'm not even sure if we are dealing with a break than I won't go into all that now.
Or are we just talking about a missing limb? If the limb is missing, your newt will heal and just have a stub. This will make it a bit akward when he is swimming and walking but will result in no serious problems in captivity.