The process seemed simple enough, but importing downloaded mbox file only gave empty subdirectories in the Local folder and no email records and no email content and no account-like set of directories shown. No error messages. I cannot imagine having done anything wrong.
I followed advice found in a couple of videos from youtube as well as some other online articles which I did not keep track of. The youtube video which I did follow was and also
Thunderbird version 45, but from Portable Apps, and the Import Export Tool extension was downloaded and installed just last night.