I've met several guys personally that have truly horrific burns due to polyester or nylon or whatever synthetic clothing catching fire - although all of them involved accelerant of some sort.

I've been wearing Nomex at work (oil & gas)for about 25 years - never heard about any health issues associated with 'em...

There is lots of good fire-retardent clothing available & yeah it's bloody expensive. Glad I don't have to pay for $300 coveralls & jackets out of my own pocket.

Many people don't realize that wool & leather are naturally flame-retardent. Both have pros & cons in the outdoors...

Synthetics also have pros & cons... quick-drying, water-proof, burn really well.

Once again I'll mention Stanfield Superwash wool underwear. Terrific in every regard. Blast; they have a one-piece, too.
No affiliation other than proud that they are Canadian company.