Look on your desktop. Do you see a folder called; Old Firefox?

https://support.mozilla.org/en-US/kb/recovering-important-data-from-an-old-profile

https://support.mozilla.org/en-US/kb/back-and-restore-information-firefox-profiles

https://support.mozilla.org/en-US/kb/how-run-firefox-when-profile-missing-inaccessible

No "old firefox" folder on desktop. Only one profile in %APPDATA%\Mozilla\Firefox\Profiles\ and it was created at 4:30 this afternoon when the computer did an automatic update while I was at work. I have searched all of C: drive and cannot find any other .json bookmark files.

On rare occasions after an update Windows might start up in a different user profile folder. Have you tried shutting down and restarting Windows?

If your old profile folder was removed, and it's not in an Old Firefox Data folder on your desktop, you might need to use a lower level undelete tool like the following:

https://www.piriform.com/recuva

Thanks the reboot worked. everything is back the way it was before. I have never encountered this weird behavior before where windows makes a phantom user ID until you reboot. I know I used the same user name I always use as it has a different password. Maybe I should switch to Linux.

I have no idea what causes it, but I'm glad you didn't spend too many hours figuring it out.

Every OS has its quirks, and often it's best to stick with the ones whose quirks you already know how to work around. In my view, you'd be in for an adventure with either Linux or Windows 10. ;-)

These can't get your data back, but will help in the future.

These add-ons can be a great help by backing up and restoring Firefox

FEBE (Firefox Environment Backup Extension) {web link} FEBE allows you to quickly and easily backup your Firefox extensions, history, passwords, and more. In fact, it goes beyond just backing up -- It will actually rebuild your saved files individually into installable .xpi files. It will also make backup of files that you choose.

OPIE {web link} Import/Export extension preferences