• 1 Download Firefox For All languages And Systems {web link}
• 2 Uninstall Firefox from your computer {web link}
  DO NOT remove your profiles.
• 3 After, reboot the computer. Then run the full installer.

FredMcD - Thanks for the suggestion... I'll give it a try!

During Windows 10 upgrade and downgrade, Windows may ask you what you want to do with your data, for example, leave it in place, save it in a temporary location, or blow it away. Probably not in those terms.

If Firefox does not automatically find your data after you reinstall it, here's the next step:

To explore, you'll first need to turn on viewing of hidden files and folders: http://windows.microsoft.com/en-us/windows/show-hidden-files

Then check whether Windows created this folder:

C:\Windows.old

If so, look for

C:\Windows.old\Users\your-user-name\AppData\Roaming\Mozilla\Firefox\Profiles

Assuming you find one or more folders there, one of them most likely will have your earlier Firefox data. If you check the file modification dates, do any of them look recent enough to be the right files?

We can describe how best to rescue data from that temporary location (Windows will delete it after 30 days).

Hmm, also, if you were offered multiple profiles to try, you could try a different one.

In Firefox, type or paste about:profiles in the address bar and press Enter/Return to load that page.

If you see more than one profile here, find one that has

Default Profile: no

and click the "Set as default profile" button.

Then scroll back to the top of the page and click the "Restart normally" button. This should exit Firefox and start Firefox up in that other profile.

Is that the correct one? If not, you could try any others available on the page that you haven't already tried.