Smoke detector for sure if you can hear it and a fire proof enclosure is great piece of mind.
I like to monitor my prints visually through Octoprint when I’m out and I have a smoke detector setup above my printer that has the red light in view of the webcam. I keep a fire extinguisher beside my printers, but that’s about my whole fire prevention setup.
I’ve heard there are ways to hookup the signal wire from some smoke detectors to a raspberry pi/smart plug setup. But I’ve never learned how to do it.
I’ve heard of people using a 'fire extinguisher ball’s above their printer, but I’ve never tried one. They seemed a bit overkill to me.