The Hebrew word was b'kowr which means Eldest son or preiminent in an order of things. First.
The Bible doesn't say one way or another what this meant if the child were an only child. Taking it 100% literally, however, it seems to indicate that the oldest of others was killed by the death angel. Can't be dogmatic on it. Just looking at the actual word though.
thanks. I read that it also included livestock.. and that there was some debate over whether girls were included.
The feast and plague is called "Firstborn." (In all the English references I've found.) You saying that's not a proper translation.