As stated on Mozilla Add-ons, Password Exporter, “Note: This add-on won’t work in Firefox 57. It may eventually if the APIs to access passwords are implemented.”
… However, this is a bit (LOL) of a problem - because, somewhere along the line, key3.db and cert8.db have become key4.db and cert9.db - we can no longer just copy the files to profile folder as we’re compromising security.
^^ The files will be read, decrypted and used, BUT it’s the compatibility version and it’s been changed in teh new Firefox; so, blah. 
EDIT: Guess the only thing possible is to use something like NirSoft’s PasswordFox to read 'em and then enter the logins in a new profile, right?!
EDIT: Btw., there’s (also) a discussion topic: So you are really killing the addon ecosystem now. Congratulations
OK SO THIS LOOKS GOOD @
https://support.mozilla.org/en-US/questions/1077630#answer-834769