AFAIK, Smartwool long underwear is 100% merino wool. No itching side-affect that I've noticed -- been wearing it a lot lately. Smartwool socks are typically 73% merino wool/22% nylon/5% elastic. Again, no itch.

Stanfield's Superwash Wool is listed as 90% wool 10% nylon.
Nothing mentioned about it being merino, so probably isn't.
It's not itchy - don't know how they do that.
It is hard to find - I usually get it at The Bay.
Stanfields does sell some items online, but not all.
http://www.stanfields.com/
Just noticed they make some FR stuff as well.

Yes, I agree with all the above, to a point.
The 3 guys I know that suffered horrible burns were in 3 different incidents and all involved accelerant.
2 were propane, one was methanol.

The methanol victim was about 12 years old & fooling around with a jug of methanol.
Methanol burns with almost invisible flame - you can't see it in daylight.
He thought the fire was out & added more fuel.
Burning methanol splashed onto his nylon shirt, which ignited & melted into his side & arm.
Years later the scars were still impressive.
Had he been wearing a cotton shirt the injuries would have been much less.
Had it been wool, maybe no injuries at all.

We could say a 12-yr old shld know better, we could say he should have been supervised more closely...
but who hasn't done something dumb as a 12-year old?

I guess my point is that if I know 3 guys who have been burned due (partly) to synthetic clothing, then the statistics must be pretty high for the general public.

Almost all my dirtbike gear is synthetic. There are 3 gallons of gasoline in the tank in front of my crotch.
We often have a little camp-fire (sometimes a BIG camp-fire)
Things that make ya say 'hmmmm...'

If you can bear the cost Nomex makes a good material for clothing. They make stuff that looks like regular cloth now as well as industrial protective coveralls.
If you need warm stuff then the fleece they make is very warm.

The thing about Nomex is it takes more energy to burn than it releases so it does not become fuel for a fire while on your body.
Even cotton canvas burns real nice once you get it lit.

I am not sure if I worry more about synthetics melting to my body or about common natural fibers catching fire and burning.
Of course a material that catches fire and then sticks to you as it burns is likely the worst.

Maybe making sure you have a couple big buckets of cold water near your fire might be a good idea.
Not only would it put a clothing fire out but the rapid cooling of your burnt hide should reduce tissue damage a lot.

three gallons? If you have an ignition with that the nature of your clothing is likely to be irrelevant....

Things that make ya say 'boooommmm'...!

I know children's night clothing is (well, used to be?) required to be flame retardant. Is this still the case? Can the same be said for adult night wear?

Russ:
I was afraid the SmartWool socks had elastic or nylon in them. I guess if you want true flame proof, you have to go Nomex. Isnt that a pain to wash though?

Nomex is easier to wash than wool, about on par with cool-max -- machine wash cold and set the drier on a cool setting; if you have the time like I usually do, just let it air dry.

I am 100% certain that anything sold as sleepwear for children in the US has to meet a standard for being flame retardant. The CPSC regularly announces recalls for products that fail to meet the standard. As far as I know there is no such standard for adult sleepwear.

I would like to point out a few things about nomex.

It's not a miracle cloth, it should be treated and washed properly, and it sure won't keep you from getting burned.

Nomex is amazing because like others here have mentioned, it doesn't melt into a pile of self sustaining plastic fire.

Used properly, (this means layers!)Nomex can provide great fire protection and mitigate secondary burns.