Lots of things are toxic in the right quantities, and I wouldn't be surprised if Black Walnut is one of them. However, I doubt you will ever experience any toxicity problems by cutting, processing, and burning black walnut. The toxic dose is probably huge compared to the very small amount of exposure you'll get while making firewood. Don't eat too much at once. I think it is a pretty decent firewood. I haven't noticed the fast off-gassing that Joful noted. There are plenty of straight Black Walnut growing around here, but lots of crooked ones too.