I think I would shy away from using anything with pine (fir is a pine.
This is because pine contains phenols, which are basically chemicals that are harmful to rodents as well as reptiles.
Here's a suggestion.. when you get your snake, first start with plain newspaper or white butcher paper as a substrate. This is really good because it'll allow you to monitor the appearances of the snake's fecal matter. Runny, really loose stools could indicate internal parasites, but these are pretty easily treated. Also, paper will allow you to see any sort of signals that the snake has mites or ticks (you would see some on the cage floor.) I would keep him on the paper for about two weeks or more, and bother him as little as possible to allow him time to get used to his new cage.
Of course, it can often be a bit stressful on them when you switch substrates, even if they've been in the cage for years.
What sub. is the snake on now?
Once you've ruled out these two nasties, you could move on to a "woodsier" substrate. Shredded aspen is one good possibility; so is cypress mulch (which I use.) I will warn you, it is often a bit harder to clean a cage with these loose substrates. And you have to look a bit closer every day to see if your snake has left you any "presents." But I think it's worth it because they look nicer and retain humidity really well during shed periods.
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2.1 Ball pythons
1.0 Boa Constrictor Imperator
0.1 albino Cranwell's horned frog
1.0 bearded dragon
1.0 Tiger salamander
1.1 breeding Clawed frogs
1.0 black kittycat
3.1 Oscar cichlids