Lava

A vanilla fluid. Feedback keeps what it does to you and gives it a second job as a fire.

In vanilla

It burns and damages anything that touches it, destroys dropped items, lights flammable blocks, and a lava bucket is a long-burning furnace fuel.

Everything above still happens. On top of it, lava is a fire fixed at 1200 Tu that never changes and cannot be blown hotter:

Under a vessel. A Crucible or Boiler placed straight over lava climbs toward 1200 Tu and sits just under it forever. It is the steadiest heat source in the game and too cool for steel (1350 Tu). See Heat.
In your hand. Right-click a lava source with a workpiece and it is heated to 1200 Tu on the spot. Crude and uncontrolled, but it needs no machine. A hot piece hurts to hold without gloves or tongs.
As fuel. A lava bucket in a Firebox burns for about 17 minutes at a steady 1200 Tu, with no variation between buckets. Lava is molten stone, so it behaves as a melt that holds its temperature while it freezes, not as a fuel that burns.