> it first clicked for me that fire burns gasses, not solids
I guess the materials would have some outgassing caused by the heat, but the flashover usually travels over partially-combusted particulate matter suspended in the air. Hotter fires have less smoke + soot, because they consume more of the fuel before it is carried away by convection. You can easily replicate this experimentally: light a candle, extinguish it, then light it again by applying a flame to the smoke emitted from the wick. [0]
Certainly for a standard in-home fireplace fire, most of the outgassing from the fuel is actually water vapour.
The fire moving through the smoke is a "rollover". A flashover occurs when the temperature in a space rises rapidly, igniting most objects in the room, more or less simultaneously.
Certainly for a standard in-home fireplace fire, most of the outgassing from the fuel is actually water vapour.
[0]: https://youtu.be/C5eTn5d0cvg