One of the fundamentally worst things you can do it place your profile folder in the documents folder. The outcome will eventually be the profile data will be lost or corrupted. Primarily this is because the very nature of Thunderbird mail database is not compatible with the AV scanning and streaming backups that are to often be found in documents folders. Like cloud backup etc

See https://support.mozilla.org/en-US/kb/dedicated-profile-thunderbird-installation This issue may be related to that.

I don't understand what you mean that "the very nature of Thunderbird mail database is not compatible with the AV scanning." Does the database format incorporate viruses or virus-like data? If it did, I would have to find a new mail reader... as would every other Thunderbird user. If it doesn't, there is no reason why AV software should mess with it.

I've been using Thunderbird since Eudora lost its support. I have kept its profile in the Documents folder from the start. I have never had a speck of trouble because of that, and I see no reason why I should now.

With that out of the way, I hope someone can help me figure out what went wrong with my conversion.

Is this a POP account? The reason you were prompted to define accounts is that thunderbird did not 'see' your old profile so it started over. If these were POP accounts, they would have started fresh, so old data would be lost. Please confirm this, and we can then help. To activate the profile, these steps may work:

if you're comfortable with doing a bit of technical work, do this: - exit thunderbird if running - click to c;\users\<yourid>\appdata\roaming\thunderbird\profiles.ini <== this is a simple text file - open with notepad or other text editor and adjust to point to the profile folder that you copied from old computer - exit and restart thunerbird - that should pick up your profile as it was on old computer

If you're uncomfortable with that, let me know.

The easy way to do this would have been to copy the whole Thunderbird folder from the old computer to the new one, as described in the following article:

https://support.mozilla.org/kb/moving-thunderbird-data-to-a-new-computer

By copying only the mailboxes, not only didn't you copy the account info, you didn't copy either the location where mail for each of the accounts should be stored, so Thunderbird essentially ignored the mailboxes you copied and everything is pretty much as if you were setting up Thunderbird anew. Only mail stored in Local Folders would have been successfully transferred that way. If you want a new account to use an existing account folder, you have to tell Thunderbird where is that folder using the account Server Settings > Local Directory.

But really, this is all a lot of pain you're inflicting upon yourself by transferring only portions of the data instead of the whole enclosing Thunderbrid folder…

I didn't copy the entire profile because when I started, I wasn't sure which folder contained the profile. There appeared to be more levels of folders in the old system than in the new one, and it took me a while to figure out the folder structure.

Meanwhile I installed Thunderbird in the course of installing all of the applications I use regularly, and setting up the email accounts was a natural extension of the installation process. So when I finished installing stuff and returned to the problem of porting the old mail, it seemed more prudent to port the local mail folders only, rather than splat a complete old profile on top of the new one and expose myself to a broader risk of compatibility issues. (The instructions I found warned me to avoid those issues by installing the latest version of Thunderbird on the old system before porting, but given the old system's barely functional state, I wanted to avoid doing that if I could.)

I thought I could locate the old system's profile relative to the inbox files. I was shaken to learn that I had nine inboxes. Eventually I realized that each one represented one email account and figured out which three represented real accounts. I never figured out what the other six were, but I deleted them without trouble.

So it's done now. I realized that the local folders no longer make sense for the way I work. I don't need one for mail relating to the move I made in 2008. But that's a problem I can deal with whenever I find time.