"Maybe all those people clinging to Windows 7 are on to something after all."

Mayayana wrote:
"J. P. Gilliver (John)" wrote
[...]

[About Thunderbird's mail storage format.]

All of that's to say that you should be able to
get your email out without much trouble, but I'm
not aware of any program that will just export single
messages. I can drag them out onto the desktop
one at a time, as can also be done with TBird. But
I can't save all of them.

Elsewhere in this thread, I mentioned Thunderbird's 'ImportExport
Tools' Extension, which *can* export single messages (in .eml format,
which is basically one-message mbox format).

If you export a folder - for example Inbox' - you get a timestamped
(Windows) folder (Inbox_20180917-1808) with a subfolder 'messages' and a
'index.html' file.

The 'messages' folder contains the messages in .eml format, for
example '20180906-Your new F1 hub-2324.eml', where 2324 is probably some
index. You can open these files with Thunderbird and they of course will
have the real Subject: sans fluff, i.e. 'Your new F1 hub'.

You can open the index.html file in your browser, which will show a
list of 'Subject', 'From', 'To', 'Date' and 'Attachment'.

The messages in the 'Subject' column are highlighted/clickable, but
they open the raw/source message. I don't know if that can be changed.

Out of curiosity I tried the
Export function in OE and it told me that it would only
export to Outlook or MS Exchange. Parochial monopoly
strikes again.

Thunderbird (basic, i.e. without Extension) can Import from Outlook
Express and - see above - Thunderbird's 'ImportExport Tools' Extension
can export to .eml or mbox (or HTML or Plain text or PDF or ...) format,
so Bob's your uncle!

FYI, I come from a UNIX and standards background, so I've always
checked if I could export my current mail folders from their proprietary
format to the only real de jure standard, mbox format. That was possible
with Outlook (non-Express) and Outlook Express with third-party software
and with Windows Mail, Windows Live Mail (barf!) and now Thunderbird.