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…