I always wear 65/35 clothing while welding, never had a problem with it melting. I don't know how it would react in a true fire situation though. I like it a lot more than 100% cotton clothing, it's more durable and quicker drying. There's also Nomex clothing if you are really concerned about it, but it'll cost you quite a bit of $$$.

While I understand the need for natural fibers for pilots or steelworkers or people who are regularly exposed to fire danger, I do think the whole "melting to your skin" fear is being exaggerated for everyday clothing. If you're in a fire hot enough to melt your clothes, you've got much bigger problems to worry about. If there is a cause for concern, you could always wear a cotton underlayer.