Hi zagestone,
The Jones fire barriers contain Boron or Boric acid which are very common and most people would consider them to be safe (see post #2 here). Of course every person may have a different answer to the question of “how safe is safe enough for me”. There is much more in post #2 here and the links it leads to may be helpful for those who want to do more detailed research into the many issues, controversies, and myths surrounding organic, green, safe, and natural materials used in mattresses
You are correct yes. Lighter weights will sink into a foam less and will feel it as firmer than heavier weights. The surface softness though which provides pressure relief should never be confused with the need for a firmer base layer to provide you with the support and alignment that you need. there is much more information about the different needs and preferences of different body types, sleeping positions, and mattress designs in the sleeping style, preferences, and statistics and in the putting the layers together (each section has an overview and includes several more detailed pages).
If you let me know your city or zip in northern NJ I’d be happy to link you to the closest “list” of better options and possibilities in the forum.
Phoenix