Fir is fine. I use a product referred to as "Orchid bark" all the time, and it is composed of fir bark. My store, Orchard's Supply Hardware, used to sell Wonderbark brand, but last week-end when I stopped in there to purchase a few more bags they had a different brand. It is now Black Gold orchid bark. Its not as red as the Wonderbark, but it still a nice product. My very favorite substrate. This is what Black Gold looks like. Just made this up today for some sick box turtles:
Just what everyone always wanted on their kitchen table, huh? The white is just reflection off the water vapor condensation on the inside of the tub: