Hi redsail,
If you are buying separate layers to put together yourself as a mattress … then a fire barrier would not normally be part of the construction and even if it was included in one of the components such as the cover you buy, it won’t be tested for the regulations as a complete mattress anyway (which is a very expensive process that is done by the manufacturers before they sell their mattresses. If you do go in this direction there would be no way of knowing if your mattress would have passed the fire regulations if that was an issue for you although this is not usually an issue for most people who are buying separate layers and making their own mattress. As a matter of fact in some cases it’s the reason people do it. In other cases they accomplish the same thing by getting a prescription so they can buy a mattress that is made by a manufacturer without a fire barrier.
If you are interested in exploring some of the very confusing and complex subject of natural and safe materials and how this may apply to fire barriers post #2 here and the links in it would be a good place to start. Be prepared for some very difficult research with conflicting information and not a lot of black and white answers.
Post #15 here talks more about the different ways to purchase a mattress both with and without the help and guidance of an “expert”. If you are going in the direction of doing it all yourself (option 3 in the linked post) with no help at all, then this can be quite a challenge and many if not most consumers wouldn’t have the knowledge or experience to be able to do this without the help of a mattress manufacturer. Many of these also offer versions of a DIY mattress (option 2 in the linked post) which may be a simpler, more effective, and less costly direction to go.
Of course each person is unique in the amount of risk they are willing to accept in a mattress purchase and some that are more adventurous may be more comfortable than others with the trial and error process of putting together their own mattress and the “adventure” and learning experience may well be worth it to them 
Phoenix