Edna is waving for a long time. She is waving when the fires are burning. She is still waving when the fires are out. For all of this time Edna is standing out on the edge, in a precarious position, when there are no flames or even smoke behind her or near her. But she stays and waves, instead of trying to escapeto the safety of the building behind her, which is not on fire for most of the time.
One video from just before the Tower comes down, shows Edna still on her perch, but something is different.