If you restored this data from an older backup then you likely had logins.json and key3.db. Firefox 58 and later use key4.db (SQLite) for the encryption key and you need to remove an existing key4.db to make Firefox fallback to key3.db.

Thanks for prompt reply. I have both key3 and key4 in the folder. I recovered from a back up taken only earlier this week. My FF version is 58.0.1 I don't knowingly use encryption. All seems to be working but is there something that may crop up that I don't know about. Should I leave key4?

key4 is now part of the two files that hold your login information. One file is for logins data, the other holds your password data via encryption.

For Firefox 58+ you need key4.db and logins.json. If for one reason you need to restore the passwords from an older Firefox version and you have logins.json and key3.db then you need to remove an existing key4.db to make Firefox fallback to key3.db and its matching logins.json file.