Start Firefox in Safe Mode {web Link} by holding down the <Shift>
(Mac=Options) key, and then starting Firefox. A small dialog should appear. Click Start In Safe Mode (not Refresh).

Is the problem still there?

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.

Firefox itself does not backup the logins, it only has the one copy which is kept as a pair of files one is a key the other is the encrypted file. If those break or are somehow corrupted there is not much you can do unless you happen to have backed them up or kept a copy of your Firefox profile.

If you did a Firefox Refesh you may still have an old data folder on your Desktop that could contain the files, or there is a possibility Windows has kept a previous version of the profile. Unfortunately it appears that feature may only be available to Windows 10 users if they have a separate additional hard disk or partition. Anyone reading this thread please take care to make sure you manually backup the current Firefox profile BEFORE even considering looking for any Windows previous version. See also

• Back up and restore information in Firefox profiles
  • Recovering important data from an old profile
•  ? partly deprecated Windows 10 http://www.tenforums.com/performance-maintenance/17767-previous-versions-files-restore-point.html
• http://www.sevenforums.com/tutorials/85679-previous-versions-restore-files-folders.html
• Refresh Firefox - reset add-ons and settings