Firefox 9 upgrade failure, previous upgrade requires reboot

For quite some time now, everytime I do an upgrade of firefox, I keep getting this popup message saying that a previous upgrade requires a reboot.  Regardless of whether I rebooted or not, the message never went away.  Typically bypassing the reboot and saying you don't want to reboot solved the problem and let me continue working.  Now with the new Firefox 9 install I tried today, it gave me that same popup during the installation process, but when trying to bypass, it just closes the installer and doesn't give any error.  From the mozilla help forums it looks like the recommended suggestion (from the top google search results) is to wipe out the whole mozilla firefox folder and do a new install.  This seems a bit over the top, so I took a few seconds to look at the folder (typically c:\program files\mozilla firefox).  Scrolling down the list of files, a name jumps out at you as something obvious.  There is a file called firefox.exe.moz-upgrade.  I renamed this file and tried the install again, this time no error.  The file was recreated at the end of the upgrade, where it prompts me to reboot.  After reboot, the file was removed during the post install clean up this time.  So I guess sometime in the past, one of those cleanup's failed.  In any case, sometimes the answer is a bit simpler than the sledgehammer troubleshooting approach (i.e. Delete a users profile, reimage a whole computer, etc).  Hopefully the new version of Firefox doesn't hang the application repeatedly for no apparent reason.