While testing the Windows profile unlocker (and during daily Fx usage since then), I've noticed that sometimes the unlocker isn't triggered in the second firefox process even though the first firefox process is clearly shutting down. I've followed along with this in the debugger a few times. From what I've been seeing, I think that the firefox process that is shutting down is still accepting DDE requests to open a new window even though it is no longer able to do so. The second firefox process sends the DDE request, assumes that the first process opened a window, and considers its job to be done. We should investigate shutting down DDE earlier in the shutdown sequence.