First things first. Ideally we would prevent Firefox from starting up automatically, but if we can't do that then we could try writing a script to automatically stop it from running after it auto-starts.
I'm not familiar with Netrunner OS but I see that it's based on Kubuntu. Do you know if there is a facility to add applications and scripts to auto-start when you log in? In Kubuntu it's in System Settings --> System Administration --> Startup and Shutdown --> Autostart
Does Netrunner OS have anything similar, and if so does it say anything about Firefox in there?
If not, please paste the following into a terminal then press Enter:
ls -a ~/.config/autostart/
Does the output say anything about FireFox or Mozilla?
With regard to whether a script to automatically stop Firefox from running after it auto-starts would work, please try running this:
Does that stop Firefox from running, and if so can you start it again without any issues?