I have heard of this feature involving smoke and fire called mantaflow. What is this feature, is it free? How will it help me with smoke and fire simulations?
mantaflow is an open-source framework targeted at fluid simulation research in Computer Graphics. via mantaflow website
If you are talking about mantaflow integration in Blender there is a branch called fluid-mantaflow maintained mostly by Sebastián Barschkis known also as sebbas. He is working on this integration for a couple of years now.
You can check also pretty long topic on Blender Artists forum where you can watch examples and read about usage of this branch.
Answering your questions, yes it's free - it will be integral part of the Blender itself and it will probably help you (I don't know what you need from fluid/smoke solver).
Basically, main goal is to switch from current Elbeem solver which is... Well. It is. To mantaflow to have more realistic simulations of smoke and fluid whith more options.