what humans were the first to use fire?

Fire probably started about a million years ago with erectus. The first signs of fire in Europe were noticed about 800,000 years ago. That happened to coincide with a warm period that melted the Greenland ice caps. It is likely that a race expanded out of Africa and became the ancestors of modern humans, some Asian erectus and Neanderthals. They probably mostly eliminated other hominid competitors.

Assuming that you mean human ancestors, it's hard to say definitively because science being science it wants direct evidence such as a hearth.

If you find a hearth what are the odds that you have found "the first one"?

There is some evidence however that about 2.5 million years ago the dimorphism between the males and females of our ancestors narrowed. It is too much of a leap to say that that means fire was used and was centralizing meat eating, but it's worth thinking about.

This is an area that we know nothing about, It probably started with lightning that started the first fire and then kept going through the tribe. Cooking meat was probably done in the same way, meat probably fell into the fire and was pulled out only after it burned. The warmth from a fire was probably recognized right away and used for this purpose right away.

The Homo erectus that migrated to the Black Sea region of modern day Georgia 1.75mya, would've had to have fire, to survive at 40 Degrees North Latitude, which has four seasons...

The two contributors above me are obviously very knowledgeable on this subject matter, but just for the questionnaire's clarification. Erectus and Neanderthal were not humans (homo sapien sapien). They were however hominids which are in the same taxonomic family as humans and share common ancestry with modern humans.

His name was George and he lived in a cold dank cave somewhere in Europe. When his cave wife, Agnes, refused to sleep with him unless he got heat for the place, George invented fire.