I have worked as a process control engineer, a quality tech, and floor labour and I have never seen something that forces such tight integration between Hardware, Software, and Process that Fused Deposition Modeling has. It is literally incredible and the open source communities ability to stay apace and integrate it all is astounding.

As was mentioned in the Basics thread the Firmware is responsible for orchestrating the movements of the machine to the instructions that are given it. The three most recent open sourced firmwares that are available for 3D printing are from oldest to most recent are 5D, Sprinter, and Marlin.

The quality of the model has a direct relationship to the ability of the firmware to accurately predict and control the processes involved in extruding, melting, and laying out the particular material you are using. This may at first not seem that much more complicated than any other CNC application and indeed the earlier the Firmware the more basic the control mechanisms and similar to a straight forward cnc control they are. However, in the effort to get ever better prints factors like the volume of filament being fed into the machines rather then just length, flow rates, amount of pressure and how it leads to ooze, accelerations around corners rather than a stop at the end of a line segment and start, how those accelerations change the amount of plastic that needs to be pushed are being considered. Additionally how hot the hotend (where the plastic is melted) makes a large difference in quality so rather than a simple on off for the heater there is a full blown PID loop. The temperature also needs to be adjusted per batch, color of plastic since the contaminants affect the optimal melting temperature. In the future real time measurements of the filament may become part of the standard firmware.

Thankfully the firmware in the Open Source community is fairly ubiquitous. There are several changes that need to be made here for the HBot. These changes are mainly due to the H Gantry design and how it differs universally from every other 3D printer currently in existence. As such every firmware that comes out will need changes to accommodate this and the more factors that are taken into account the more code these changes need to take place in.