[https://superuser.com/questions/603080/import-passwords-stored-in-csv-file-to-firefox This] seems related. Maybe one can install Firefox ESR and use some legacy plugin that allows some XML... import.

You would probably have to install Firefox 52.6.0 ESR in a separate directory and use the Password Exporter extension to see if that allows to import the passwords. Make sure to use a separate profile for Firefox 52 ESR. *https://support.mozilla.org/en-US/kb/profile-manager-create-and-remove-firefox-profiles If that succeeds then you can copy logins.json and key3.db to the Firefox 58 profile folder. Make sure to remove and existing key3.db. If you need to merge with existing passwords then it gets more complicated because Firefox 58+ uses a key4.db SQLite database. You would have to set Firefox 52 to use SQLite as well by setting an environment variable. * export NSS_DEFAULT_DB_TYPE=sql In that case you need to copy logins.json and key4.db. *https://support.mozilla.org/en-US/kb/recovering-important-data-from-an-old-profile