I found this text/video guide from IGN that will be of some use for those who prefer text guides. (and even video)
http://www.ign.com/wikis/batman-arkham-knight/The_Line_of_Du... But if you prefer not to go to an external link, here is a video that will work:
NOTE: In order to save the 15th firefighter, you will need to get the bat mobile into Founders Island. (Unlocked via story) And for the 16th firefighter you need to rescue the other 15 guy for his location to appear.
Hope this helps! If you have any questions or comments let me know.