Mark, two things to try.

You might be able to restore user data that has gone missing by following the steps in this article: https://support.mozilla.org/en-US/kb/recover-user-data-missing-after-firefox-update

If you go to Tools menu / Settings and search in the search field for "Always check if Firefox is your default browser" and uncheck it, hopefully this might stop the popup requests.

Thanks for this advice. It solve part of the problem. I had three profiles: Default, Default-Release, and Default-Release-1. None of them had my saved passwords. However, the two Default-Release profiles did allow me to start rebuilding the password file by prompting me when I logged in to a new website. I also noticed that with the Default profile, there is now a little key on the left side of the search window that, when I click on it, lets me add a password. So, that's all good.

I am still getting a new private browsing window every time I try to open a new tab. I have no idea what that's about.

I also, of course, still have no idea why all the passwords disappeared in the first place, ane little confidence that they won't disappear again. Also, I don't know why I kept having to restore FF as my default browser. But, for the time being, I can manage around those things.

Thanks again.

Mark, another thought. Sometimes missing passwords can be retrieved using the logins-backup.json file in the Profile folder on the hard drive. But this should be attempted as soon as possible to avoid any remaining passwords getting overwritten.

1. Fully exit Firefox if it is running.
2. Open your main Profile folder on the hard drive.
3. Move logins.json to a backup folder.
4. Copy logins-backup.json to a backup folder.
5. In the main profile folder, rename logins-backup.json to logins.json.
6. Restart Firefox.

If the backup from the main profile folder does not contain the missing passwords, try the backups from the other folders, remembering to copy them into the main folder and rename them.

If this doesn't work, then you can always exit Firefox and restore the original two files that you initially backed up.

Regarding the other problems, I am not sure about the private window issue. Perhaps you could start a new question and see if someone else has the answer?

If either you follow the previous steps and recover your passwords, or you just rebuild them from scratch, you can back up the two files "key4.db" and "logins.json" from your main profile folder on the hard drive (while Firefox is closed, naturally).

Firefox stores all your password database information in these two files. So if necessary in the future you can fully close Firefox and copy these two files back into your profile folder, and their stored passwords will be restored.

For more information on the individual files within a profile folder, see this article: https://support.mozilla.org/en-US/kb/about-your-important-data-and-their-files

One last thing, if you go to Tools menu / Settings then on the left hand side choose "Privacy & Security" and scroll down to "Passwords", then you might be able to select options there for asking you to save passwords for a site etc.

Thanks again for all your help. I am familiar with .json files, and I will try what you suggest and see what I find there. If there's nothing, it's not a big deal to rebuild the file as I enter new passwords, and I will back them up outside of the profile as you suggest.