Hi, first try restarting Firefox - 3-bar menu > Exit Firefox in bottom right corner.

If that doesn't help, you can try this: Open bookmarks > Show All Bookmarks > Import and Backup > Restore, and see if you have a backup there.

If not, please see -

• Profiles - Where Firefox stores your bookmarks, passwords and other user data.

• Recover important data from an old profile.

If your question is resolved by this or another answer, please take a minute to let us know. Thank you.

Thanks Scribe. I'd already tried these, or did so after my post, with no result. However, my problem is resolved. I found an option to revert to Windows 7, within 30 days, which I activated. Worked like a dream with everything back unchanged including the Desktop, all files, email and my Firefox bookmarks. I'm relieved and happy, and will be more cautious with updates in future.

For future reference...

The Windows 10 update has various options for what to do with your old data. One of them creates a folder named C:\Windows.old and moves your Windows 7 application data there. That's probably what Windows created in this case, or it would have been very difficult to roll back the change.

To use that folder, 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 here:

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

Assuming you find one or more folders here, one of them most likely will have your latest pre-upgrade settings.

Many thanks for this information jscher2000. I'm residually hacked off with, I guess, Microsoft for what happened. There was no obvious warning about the loss of information and system functionality that would occur in upgrading to Windows10. This went well beyond Firefox and resulted in several hours of useless endeavor and frustration. I could find no way of communicating with Windows10 about the loses. Perhaps I should have realized that changing the operating system is no simple matter.