Firefox does not have a feature to undelete passwords.

Do you back up your Firefox profile folder containing your settings files? The file you need to find/recover is logins.json.

If you do not have a backup, on Windows 7, you could right-click the file, call up Properties, then the "Previous Versions" tab to see whether a restore point contains a copy of the data.

On Windows 8+, this might not be enabled by default, you might need to turn on the File History feature. However, I posted a possible workaround here: https://support.mozilla.org/questions/1012716#answer-608899

Note: You can open your current Firefox settings (AKA Firefox profile) folder using either:

• "3-bar" menu button > "?" button > Troubleshooting Information
• (menu bar) Help > Troubleshooting Information
• type or paste about:support in the address bar and press Enter

In the first table on the page, click the "Show Folder" button.

If you still have the signons.sqlite from the previous Firefox version and you haven't changed the Master Password so key3.db is still valid then you can try to re-import the passwords.

You can force Firefox to reimport the passwords from the signons.sqlite file.

• reset the signon.importedFromSqlite pref on the about:config page to the default value via the right-click context menu
• delete the logins.json file in the Firefox profile folder with Firefox closed

When you restart Firefox then you should have the signon.importedFromSqlite pref with the value set to true and you should have the passwords imported in the Password Manager unless there may have been errors.

You can use this button to go to the currently used Firefox profile folder:

• Help > Troubleshooting Information > Profile Directory: Show Folder (Linux: Open Directory; Mac: Show in Finder)