Um, no, if you really believe that people who
regularly have to risk their lives by going in burning buildings and whatnot decided to change their fabric of choice
just because of marketing, a reality check might be in order. :v
Here's a brief layman-level explanation as to why it goes beyond simple flame spread index. A modern firefighter suit isn't something you can replicate in leather. Even if Nomex and other high-tech fabrics didn't exist, alternatives to leather would still be actively sought out because of the seams (made chiefly out of cotton threads, which are remarkably bad at surviving repetitive exposure to extreme heat) and the fact that leather is as good at keeping heat outside as it is at
trapping it inside. If you've ever tried wearing a pair of leather pants in hot summer... well, the sensation is certainly hard to forget.
Actually, this hasn't been the case since about forever, but a
good non-cotton shirt is neither cheap nor easy to find, which is why people tend to forget they exist at all. Out of natural fibers, linen and hemp are much,
much superior to cotton in every respect except wrinkling; certain types of wool and silk are also more breathable, stretch and stain less over time, and are far less odor-retentive compared to even the lightest cotton. I have clothes made out of all of these fabrics, and cotton is by far the worst at wicking moisture, body heat, and odor away. (That being said, I also avoid cheap synthetics like plague.) Denim jeans are also 100% cotton, by the way; certainly not something I'd mention with "breathable" in the same sentence. In any case, give 100% hemp a go if you can procure it—I guarantee you'll love it.
You might also want to check what professional athletes choose when money is not an issue. Modern synthetic and mixed-fiber fabrics are extremely versatile, and can be tailored to mimic virtually any characteristics you want if your equipment is good enough. E.g. polypropylene is used for hypoallergenic babies' diapers, dishwasher-safe tableware, ultra-absorbent microfiber cloths, and waterproof tents, and is good at all of those applications. Fascinating stuff.
As for non-clothing fireproofing, I suggest taking a look at
aerogel.