I am not sure if this behaviour can be recreated but this is the scenario I had.
I had Firefox 0.8 installed (I think by way of setup), and when 0.9 came out today (relatively), I downloaded it (i.e. the selfcontained zip version - not the installer) and extracted it to my previous firefox directory.
And just in case you wondering it was a "yes" to replace all files.
What resulted was a behaviour that I call the 'firefox dance'. Running the new firefox exe causes it to repeatedly kill itself and respawn a new firefox.exe process. <- Please note emphasis on repeatedly. Only way I could get out of that damn loop was by restarting the machine.