https://kb.mozillazine.org/Thunderbird_:_FAQs_:_Recover_Deleted_Account

Thank you for your help, but it didn't work as every attempt at accessing the (local) inbox fails because TB tries to login on the server side, and my account doesn't exist over there anymore. What I think I need is a way to read the local nstmp files.

That sounds like your account was an IMAP one. With IMAP there is an option to download your messages from the server for offline use. In Thunderbird speak that's called 'synchronization'. https://support.mozilla.org/kb/imap-synchronization

Did you turn synchronization on for your account? If yes, your mail would be in files without a file name extension, e.g. like Inbox, and your messages would be visible. You could start Thunderbird in offline mode to avoid attempts to connect to the server.

If not, then there is nothing to show, since no offline copy of your messages exists, and the server isn't available anymore.

All the data is still local, in my profile

I wouldn't bet on that.

I can see a partial preview of the messages when I run a search in TB

That's most likely data from the index of the Global Search database. It doesn't contain the full messages though. https://support.mozilla.org/en-US/kb/global-search

What I think I need is a way to read the local nstmp files.

Nstmp files are leftover files from failed compacting events. I doubt that you can recover messages from them. https://kb.mozillazine.org/Files_and_folders_in_the_profile_-_Thunderbird

What's the best way to read those ghost messages ?

Lacking an offline copy of your messages, your best bet would be a backup you created prior to deleting your account. If I'd have to guess, you probably didn't create one.

Sadly, you are right on every account. Thank you again anyway !

I was able to retrieve an old version of that profile, with the INBOX, SENT... files. I copied it while offline in my current profile, but the emails still don't show. Is there a how-to somewhere that would explain how to read from a 26GB INBOX file?

Edit : never mind, I found the solution here : https://support.mozilla.org/en-US/questions/1295495

The first thing I'd try is to turn off your anti-virus real-time scanner. The most reliable way to do that is to start Windows in safe mode. Try to start Windows 10 in safe mode with networking enabled.

Does the problem go away?