When I first started using the Firefly you could say I had to learn not to combust rather than learn to vape from the Firefly.
I have continued to refine my technique to the point where I no longer feel the need to "pull as hard and fast as I can". I can now pull at a reasonable rate and still get satisfying results.
I've had a few charred bits in my ABV, and experienced some "burnt" flavor in my vapor, but I would say it is far from
combustion. I've experienced combustion in my T1 and MFLB (actual flames visible in the trench), and this is no where near that.
The first hit produces thin cool vapor. Subsequent hits get more dense, but none feel like they are approaching combustion.
I'll be honest, I think I would prefer a unit that does not approach combustion. Nothing kills "stealth" like the smell of combustion coming from a vape.