Factual error: During the Christmas season (December) Travolta and Phoenix discuss a transfer to an aide's position. That same night Phoenix discusses the possibility with his wife - with crickets chirping away in the background. Not in Baltimore in December.
Factual error: Before Jack starts to rappel down the side of the building to save the man, he tightens his carabiners as tight as they go. Anyone who has undergone training (assuming that a fire fighter was properly trained) would know to twist the carabiner nob a half turn back, otherwise after climbing the nob would be so tight that it would literally take the "Jaws of Life" to get it open.
Answer: I'm not a fire fighter so I do not have a answer based on experience but I assume that when Jack was disoriented it's because there was no light in the building, then when the fire broke through it produced enough light to break through the smoke and make seeing easier for him to escape.
It actually doesn't break through the floor...it's a flashover where everything including the gases in the smoke reach their ignition temperature simultaneously and the room lights off...it's a very serious and dangerous situation to be in as a firefighter...I was in one, luckily we were only 10 feet inside the door.
Steve Kozak