It is not firecontrol that does that…the underlying control board does that. In my pro it is GRBL controller. I have done exactly what you want when I set up my 4 axis. I use the X axis for it and to get the rotary spin right, I have to reset the x steps…https://www.diymachining.com/downloads/GRBL_Settings_Pocket_Guide_Rev_B.pdf
So I reset the $100 command to the correct steps/mm.
Enjoy the rabbit hole and before you mess with it — make sure you copy the current parameters that Langmuir has set up for your machine.
I would suggest you download something like “Universal GCode Sender” to play with this some. It will allow an environment similar to what you ask for in that you can send GCode and update GRBL commands from the interface.
Having said all that. I love firecontrol. Not perfect, but does a whole heck of a lot.