All case law may vary from state to state, but from what I've read so far I will add my .02 as a LEO (and a FF/EMT-IV).

The FF's (in my opinion) were acting under their job capacity, but they were not acting upon behalf of law enforcement. If they extinguished a fire and were conducting overhaul on a home and discovered any incriminating evidence, it would be admitted all day long in our courts (in TN). This is obvious in previous court rulings involving homes with meth labs. Our courts are not going to throw out evidence of a meth lab just because FF's found it during firefighting operations. It's one thing to find it while fighting fire, it's another thing to enter the home illegally.

However, overhaul procedures would not usually include opening boxes that are found. One must look at a couple of things #1 - Did the FF's have a right to be there? Obviously the answer is yes, to preserve lives/property from fire. #2 - What right did the FF's have to open a box? Was the box at any time considered a threat (as far as on fire or catching other things on fire)?

If any item (drawer, box, etc) is not on fire and doesn't pose a risk to any emergency personnel on scene, you would have a hard time explaining why you are sticking your nose in it. The owners of the home can rummage through their own belongings once the fire threat has been eliminated.

My standpoint comes from general law enforcement search & seizure as a police officer. I have no training/experience in arson investigation so I will leave that topic to others that have. My advice above is based on basically anything else that's found during a fire that is illegal (drugs, bazookas, whatever).