It's not a stupid question, the earliest utilization of fire is a really fascinating question that archaeologists and paleontologists are trying to answer. This is because the utilization of fire allowed for cooking of food, which is a significant gain for early humans, and likely contributed to increased brain size- leading to modernity in our species.

To answer your question- "the greatest discovery in the history of mankind is fire" is an expression. To be specific, the "discovery" of fire occurred in prehistory. In light of what I mentioned above, I think the important innovation in relation to fire is the utilization of fire. Humans didn't "discover" fire, they most likely encountered it as a natural phenomenon somehow (e.g. lightning strikes, wildfires, accidentally causing a spark when banging rocks together). The more important innovation is the use of fire to cook food, or the purposeful making of a fire for warmth. Making fires for warmth would have been one of the early forms of climate control, and may have led humans or our ancestors to inhabit colder places.

As ever my favorite book for paleoanthropology (study of human evolution and early prehistory) is The Human Career by Richard Klein.