White fiberglass looking material inside fireplace - what is it?
There are many non-asbestos high-temperature fiber insulations (Kaowool, Superwool, rockwool, etc.) - but as with all asbestos questions, you can't tell by looking, so send a sample to a lab if it's of an age where asbestos is possible.
Fiberglass won't take the heat, so it's not that.