Unfortunately, unless you have a backup of your locally saved logins.json and key4.db files from recent weeks (or whatever you might be willing to roll back to), I don't think there will be any way to recover that password.

Can you reset your FB password on FB's site? They have a help article on that here:

https://www.facebook.com/help/213395615347144

You can check if you have a backup file named logins-backup.json in the Firefox profile folder that allows to undo the last change made to a login. That would require to rename logins-backup.json to logins.json to replace the current logins.json. Make sure to keep a copy of the current logins.json file. You can possibly open this logins-backup.json file in a Firefox tab to check if you see a Facebook item in the file (name and password are encrypted, but you should see the URL in plain text).

• https://developer.mozilla.org/en-US/docs/Tools/JSON_viewer