Here are my thoughts...
There are times when its really dark. The on-scene lighting may be limited to close to the apparatus. However, the fire itself often provides some lighting. In these cases, don't be afraid of high ISO, wide apertures and show shutter speeds. Really concentrate on holding the camera still when the shutter speeds drop.
Here's a photo of a shed fire before any on-scene lighting was setup. The fire itself provided what lighting there is. This was ISO 1600, f/4 at 1/60.
This one was my limit at the time in terms of ISO and aperture, and even so I was still only able to get 1/50 shutter. ISO 3200, f/1.4 at 1/50. There it a wee bit of on-scene lighting - the green in the tree just right of center is illuminated by the fire jeep's headlights.
Another case of little on-scene lighting. The only illumination here is from the fire and the jeep's lights. ISO 1600, f/2.8, 1/30
Using flash in these situations can give a vastly different look. ISO 3200, f/4 at 1/60
When the light allows, I generally tend towards higher ISO and no flash. Not always, but generally. This one would have had a very different look with the foreground brightly illuminated and the reflective piping blown out. ISO 1600, f/3.2 at 1/200
Here are a couple of similar photos of a fireground scene - the first without flash and the second with flash. In this case, there was a fair amount of on-scene lighting.
Without flash, ISO 3200, f/2.8 at 1/60
and with flash, ISO 3200, f/2.8 at 1/100. You can notice a different in the foreground and the reflective piping.
I think I'm about at my photo limit for a post, so I should probably wrap up. A few other things that come to mind...
* when using flash, I often dial in somewhere around +1 FEC and allow the reflective piping to blow out. Otherwise, the reflective piping will be ok but everything else will be darker.
* on some scenes, using M is darn near impossible (unless using flash) because the lighting changes so much (e.g. how active the fire is in the scene, whether the scene you're shooting is illuminated by a flood or not, etc.).
* Flash makes flames much smaller. When using flash you'll tend to see less of the flames. Often the flash will illuminate whatever is behind the flame and where the flame isn't very bright it will be overpowered by its background.
* I always prefer noise to blur - so I'll max the ISO before going to a really slow shutter speed.
I'm still learning myself and I'm sure I've made some mistakes in the above. And I know I have plenty more to learn, but experiment and have fun.