I know it's not considered safe to rappel down a building, but I'm fairly small and I have a better chance of surviving trying that than staying in the building.
Jesus!! People here have some pretty dramatic fantasies.
Highrises aren't all that vulnerable to fire. They have very good firewalls. So as a precaution, you'd really only have to go down a few floors to be safe. There was a fire in an apartment in a New York skyscraper about five years ago. It looked dramatic, but it was confined to just that apartment.