Hi Ironist,
Congratulations on your new mattress … I think you made a thoughtful and great quality/value choice 
I hope you have the chance to share your feedback when you’ve received it and had the chance to sleep on it for a bit.
You can read a little more about fire retardants in this article and in post #2 here. I’m not so sure I would be considering a natural fire barrier for its own sake in a mattress that contains memory foam and polyfoam which are not natural materials but of course it would depend on the specifics and the fire barrier that the “regular” Katherine uses and what they would be replacing it with and the overall effect it would have on the mattress.
Post #89 here has more about the pros and cons of different types of mattress protectors.
Phoenix