Two separate issues, Milo! I recovered the profile (phew!) but its apparently possible loss was consequent upon following the diagnosis procedure you sent me for the main issue, which was the false and uncontrollable elimination of newly incoming e-mails by Thunderbird which was (very, very) probably linked to its ignoring of my chosen global junk settings and also those for each individual e-mail account.
The faulty junk handling, for which you kindly sent me the Mozilla Diagnosis procedure etc., remains exactly as before, even though I followed the diagnosis procedure to its end which required the uninstalling of Thunderbird and the installing of a brand new copy (which is where the backed-up profile wouldn't come back - now solved).
And you say that you're confused!