Every plane is equipped with a fire extinguishing box (for lack of the proper term) where you drop your battery and gets extinguished, air crew are trained on it. If you don’t have any warnings on the cabin, telling you that other systems are failing, you do not declare an emergency, you declare a fault (PanPan)
757 has a stellar record, but as you say, it’s showing its age. And that means more paperwork on Boeing releasing proper maintenance procedures, revising inspection intervals, releasing TNs, etc. which are mostly on program management, an area where Boeing has been lacking lately…
At no point I draw any conclusions, I am just putting in doubt (which is nothing bad, is just not assuming something as by default given) that Boeing has been doing everything by the book. And of course that’s something that will be investigated by the FAA, and I hope they have been doing everything well :)