Seriously, we do know quite a bit about this already. It is harmful, it has been proved to cause issues in humans and other animals:
http://envirocancer.cornell.edu/pbde/brief.cfm
and one directly involving fish (the references alone are worth reading):
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2174405/
If you want to use it, fine. Be my guest. However, I do believe this is important to note that many of the companies are even coming out and stating they will be using different ways to make sewing materials safer for the consumer and the environment.
All I know is that I would get really congested after using certain quilt batting. I stopped using the flame retardant ones and I feel fine. Proof enough for me and several other quilters that were kind enough to let me know this could be the cause. The plus side is they are getting pressure to removal chemicals from the battings and have come out with alternatives. If it says flame-resistant, you are putting your fish at risk. And this it didn't harm my fish or claiming we don't know enough about it when scientific evidence points to the contrary, doesn't make it safe. Just ask people in Meigs county, Ohio what perfluorooctanoic acid did to them and the environment. People like to dispute C8 just as well despite overwhelming evidence.
However, if you want to use it, be my guest. It's your tank. But if I were breeding or keeping expensive fish, it's something to think about and research.