You're talking about those bendable bridges? My rats absolutely love them. I like them too because they are very inexpensive when it comes to replacing them.

I did some research;
'China Fir' is just an easier name for a type of evergreen tree, not a fir. It's a softwood. Most small animal chews are made from softwoods. People suggest that any presence of softwood in an animals habitat is bad because they contain phenol's. The same article also suggested that most other types of woods are toxic to small animals too. The author of that article was a general journalist, not an expert, and there were no sources.
Another list I found on About.com, written by an 'exotic animal expert' listed many types of woods. Apple and kiln-dried pine wood were the only regularly found types of wood that she suggested to be safe for rats. The unsafe list contained cypress (which is the family that 'China fir' is part of). Then at the bottom of the list it says that there are a verity of safe woods that can be found at pet stores. But AGAIN there are no sources, that really bugs me.
Lastly, if you look up the poisonous plants list on the Rat Reprot you will find 'Chinese Evergreen' lifted under toxic plants. However, this is not an umbrella for all evergreens that are produced in China, this is a specific leafy plant.

So, it looks pretty inconclusive. If you really want to keep your rats in a bubble you might not want to buy them Petco chew toys. However, I personally think that they are fine. My boys have enjoyed sleeping under their bridges and occasionally nibbling on the edges. I never thought anything of it.

 

This is the list I usually go by when buying wood for my rats. http://exoticpets.about.com/od/RatSupplies/fl/Safe-and-Toxic-Woods-For-Rats.htm

I don't see China fir on the list but as was stated above if you mean those little bendable bridges then I have been using those for a while with no adverse effects and my rats enjoy them.