I can't help but wonder if those early firestarters discovered the sparks from pyrite and flint through bored fidgeting with rocks, or if they were trying to do something else when they saw the same sparks that could spread fire over small barriers and gaps.
Probably the latter considering how often flint shows up in paleolithic technology.
If they where making flint axes I could guess that since that involves beating one rock with another they probably found that some created sparks and if such a spark set a leaf on fire that could very well be the discovery.
Trying different rocks to find the ones that makes the most accurate edges would be a quite natural thing once you found that hitting flint with some other rock would cause it to chip and create a sharp edge.
And it would explain how it might be discovered in may places independently and also who a tribe isolated from others in an area without the right kind of stone might either never discover fire, or maybe forget it after enough generations in isolation, especially if they had enough supply of food and where in a climate where fire for heating was not really required.
