Fir is fine, although it will break down over time.
As a comparison, I have black plastic composters, one 35 years in constant use and others of various ages - but all more than 20 years in use.
I also have raised bed sides in constant contact with soil; these have been completely replaced at least 3 times in 35 years, with plenty of little repairs here and there besides the complete replacements.
For me, having the light weight and reliability of the plastic is a priority. BUT there is no compost process nor quality reason that fir would not be equal to or superior to the plastic.
It's a matter of choice. However, DO NOT use wood that has been pressure treated or dipped in any kind of preservative.
This work is supported in part by New Technologies for Ag Extension grant no. 2020-41595-30123 from the USDA National Institute of Food and Agriculture. Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture.