Just a couple of thoughts from a Firefighter....(who is also a photographer)
As long as the camera is the same relative size as the "regular" helmet cams that are out there, it should not be an issue as far a wearing it, What will be an issue to getting it to survive in the fire environment. I would suggest that you work very closely with the folks who are going to wear it and I am sure that you/they can figure a way to protect the camera.
I have seen an idea of using the "hood" that we wear to help protect the camera. That may protect it from the heat, but you are also going to have to protect it from smoke, steam, and impact.
The other issue that you are going to have is visibility. Most of what we do in a fire is done in the dark. With all the smoke etc there just isn't a lot of light available to capture. You may also have issues with where the camera is pointing as it may be looking at the floor or the back of the firefighter in front on the camera. Even in the best of conditions with a well ventilated fire, visibility is low at best.
The other option you may want to explore is to film crews at a local Fire Academy. These fires are much more controlled and may offer a better "perspective" for the fire and be done in a "less stressful" situation.
Remember that what you see in films is nothing like what the "real thing" is like. If you have any specific questions, please feel free to let me know and I will answer them to the best of my ability,