Fire based furnaces replaced fireplaces and have been heating homes for decades. The earliest types of furnaces were not "forced air," meaning there was no blower fan to move the air in and out of the furnace. Instead, density and gravity helped draw in the cold air and force the warm air into the house. This, of course, was highly inefficient.
Forced air furnaces were eventually introduced but the current high efficiency models are more efficient that models used in the 1970's. This is not because of any change in chemistry or physics. A flame fed by natural gas burns no hotter now than it did in the 1970's. The difference is with the engineering of the furnace itself. Modern units can be up to 95% efficient. Older models lost heat in the heat exchange unit through the ventilation pipe with the vent gasses. Modern units have a completely airtight sealed heat exchange unit that does not allow the hot air generated to escape out the exhaust. In addition, there are secondary heat exchangers installed to capture any heat lost from the primary one. These improvements in engineering help modern furnaces to heat the same volume of air to a hotter temperature using less fuel than units from the 1970's.