There are several people who have had bad experiences using a fir bark substrate. Their snakes ended up having seizures, and one even died. It is not confirmed that this was actually casued by the fir substrate, but once it was removed the issues stopped.
If you want a natural looking substrate, aspen shavings, Eco Earth, or coconut shavings or bark are the most widely recommended, as they definately do not cause problems.
Why take a chance?