In the event that the passwords information is lost, you should record all information in a separate text file somewhere else on your hard drive, or written down. You can easily copy and paste if you need to do so. If you are concerned about someone else looking at that file, you can compress it using a password.

Back up and restore information in Firefox profiles.{web link} I use an add-on that makes backups of my user profile, Just In Case. Unfortunately, it is not compatible with the current Firefox.

Every now and then, open the profile folder and create a copy of the current profile as is to another location on your hard drive. This way you will have a backup.

If you do this each day, you will not lose much data. This should be done with Firefox Closed.

Note: In case you need to restore from these backups, you may need to overwrite old or corrupted files.

ktorpil said

. . . . and it asks me to reconnect to sync

You may be getting logged out when the browser is closed. If you follow the instructions above, that should take care of it.

All my saved passwords are also gone

Look on your desktop. Do you see a folder called; Old Firefox? Look inside. Look for the folder with the latest creation date.

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

Firefox stores the credentials to connect to Sync in the Password Manager, so if there are issues with the passwords then you can expect issues with Sync as well.

What security software do you have?

There have been reports that AVG/Avast is causing issues with passwords disappearing. You can check the profile folder to see if there are logins.json.corrupt files present. If you find such a file then try to rename to logins.json.

• keep a backup copy of all logins.json files

You can use the button on the "Help -> Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the about:profiles page.

• Help -> Troubleshooting Information -> Profile Directory:
  Windows: Show Folder; Linux: Open Directory; Mac: Show in Finder
• http://kb.mozillazine.org/Profile_folder_-_Firefox

It seems to be solved now.

I uninstalled Firefox and deleted the Mozilla folder at %APPDATA%\ . After I installed Firefox again and logged in to sync my passwords are there again. I restarted my computer and there were no problems.

I backed up my profile directory from another computer that I use and was going to copy it over but it seems unnecessary now.

Are you using AVG/Avast security software?

cor-el said

Are you using AVG/Avast security software?

Yes I'm unfortunately using AVG Ultimate

If you use AVG then you possibly had an issue that AVG had caused logins.json to get corrupted or Firefox couldn't access the file (i.e. AVG has locked it) and Firefox renamed the file to logins.json.corrupt like I posted above. We have seen several reports that all mention AVG/Avast security software.

cor-el said

If you use AVG then you possibly had an issue that AVG had caused logins.json to get corrupted or Firefox couldn't access the file (i.e. AVG has locked it) and Firefox renamed the file to logins.json.corrupt like I posted above. We have seen several reports that all mention AVG/Avast security software.

I am 100 % sure that it happened right after I updated my Windows 10 to version 1903

ktorpil said

I backed up my profile directory from another computer that I use and was going to copy it over but it seems unnecessary now.

Back up and restore information in Firefox profiles.{web link} I use an add-on that makes backups of my user profile, Just In Case. Unfortunately, it is not compatible with the current Firefox.

Every now and then, open the profile folder and create a copy of the current profile as is to another location on your hard drive. This way you will have a backup.

If you do this each day, you will not lose much data. This should be done with Firefox Closed.

Note: In case you need to restore from these backups, you may need to overwrite old or corrupted files.