Hi once again, chaps.
1. It was Thunderbird which I uninstalled, not Firefox, and I did it following the instructions in the Mozilla Standard Diagnostic which you kindly sent me, Milo. It specifically says, in capitals, that the uninstall will NOT delete any data or profile material.
2. My e/mail is POP and it-s with Terra - a division of Telefónica which is the principal national telephone company here in Spain.
3. I'm not certain that the e-mails weren't preflagged as spam by the server but they weren't spam and it has never happened before. Also, there have been issues with junk posted on some Mozilla fora.
4. XP Home
5. Microsoft firewall, Avast free, Spybot, and sundry other stuff like BetterPrivacy, AdBlock Plus, Ghostery, and some anti-tracking stuff too.
I saw the profile being backed up over some time, with many of the file and folder names flickering up on screen as the process was being carried out. I saw the name of the backed up profile file (but didn't write it down, thinking that Mozilla could surely be trusted to handle something so fundamental yet routine). I thought of using Recuva to look for it but decided to talk to you all first before interfering more.
This is really serious now because, for sure, the profile back up will have overwritten my previous back ups so I will have lost ten years of business and personal mail.