The article mentioned a database the Times was using to track the location of firefighters throughout the incident, I looked at that system. It is not publicly available but was interesting in that it entirely depended on the news footage, personal videos and eyewitness accounts.
The article mentioned helicopters were being considered for rooftop evacuation, which surprised me due to the smoke. I looked into the visibility in the airspace directly surrounding the World Trade Center, while the smoke was so severe as to be captured by the International Space Station the wind did clear the smoke sufficiently for a clear line of sight between the helicopters and portions of the towers.
The firefighters were climbing the stairs, and reportedly would take hours to reach the top. The article also mentioned several groups of resting firefighters, so I researched the weight each firefighter would have been carrying which added up to 45 pounds of gear and at least an additional 20-30 pounds of tools.