I realize this is a very late reply lol, but for other people wondering to know I found this out about fir bark.

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Bedding choices consist mainly of aspen, coconut bark or fir bark. My choice is coconut bark in my display tanks and aspen in my breeder racks. I have heard that Fir Bark can carry mites and must be heated before use ( I have never seen this but thought it was worth mentioning). NEVER USE PINE OR CEDAR SHAVINGS as the phenols they release are toxic to snakes. Many also use newspaper, paper towels, and Astroturf which are all acceptable substrate but on the ugly side."