What about the rest of your profile? Settings, bookmarks, history, passwords . . . .

I had to change the password for the account in question. After the change I tried to sign in with the saved login but it was gone. Now, every time I want to sign in I have to add the login in and password manually. Nothing else has changed.

I suggest you reset the password again. And create a new password, and write it down.

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.

Do you still know this password and do you want to save it again in the Password Manager or have you forgotten this password ?

If you use the Time Machine or other backup then try to restore the logins.json file in the profile folder from this backup.

• http://www.apple.com/support/timemachine/

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
• https://support.mozilla.org/en-US/kb/profile-manager-create-and-remove-firefox-profiles
• https://support.mozilla.org/en-US/kb/recovering-important-data-from-an-old-profile

Problem solved.

Can you tell us how you solved it as this might help others with the same problem.