@aimee
Firefox is bulky and takes time between giving up its window until releasing its last file and unsetting its lockfile.
If it waited before giving up the window, people would sit there clicking the close button until it did, thinking there was a problem with it. If it brought up a courtesy window people might think it strange/unnecessary/a waste of resources or whatever.
It looks like you're caught up in the 'lowest common denominator' so to speak.
P.S. This is one of those subtle thingies that makes people try Opera