I think that the answer to your question depends on how you intend to work with the messages after you have moved them.

You could remove the underlying data files but they would be pretty useless to you. You would have to add them back into Thunderbird to make sense of them.

If you use the default "save as file" option then you'll get eml files, which need an email client to open them, or html files. In both cases, you lose their connectedness, so it will be hard to find the various messages that make up a conversation.

I'd suggest the ImportExportTools add-on which can do both of the above and build an index which might help with locating content. IIRC it may also offer a "Save as pdf" option which appears to appeal to many users, though I must say it seems a clumsy solution to me. I think html gives you the best option for a searchable archive.

Thank you for your reply Zenos .. but if I understood any of what you said I would have gone ahead and done that. Without "over-thinking" this... all I need is a name and their email address that I can read - once I move them out of the Local Folders. The Local Folders are getting crowded.. so I need to move them out.

A "point and click" is the only thing I understand -- I don't speak "computer-language" very well - so point me to where I need to go and tell me what I need to click on .... thank you!

If you select then drag-and-drop any message to your Desktop, or to another folder, it will create a standalone file storing that message. You can then delete the original message. In fact, you can select several messages at once using shift+click or ctrl+click and then move them all in one drag-and-drop action.

Unfortunately, this drag-and-drop is a "copy", not a "move" but so long as you are methodical, you'll find that when you go back to Thunderbird, the messages you just copied are still highlighted and so can easily be deleted.

The structure of Thunderbird doesn't let you do this with folders; you have to work with messages. This where the add-on might be more useful.

what I found was.. by creating folder on hard-drive I could "h-light the 'block' of emails for a particular person, etc. and drag&drop them into the folder on the hard-drive. I found I need to "re-name" them to recognize the emails.

I could then go back and delete the 'sub-folder' in the Local Folders.

Mission accomplished without relying on some unknown "APP"/add-on (?)

Now that I explained what I did it was the same as what was instructed - MOVE in my OLD dictionary is the same as C&P today!

Thanks.. again - Terry

It's not an "unknown app". I'm recommending it to you.

A core strength of Thunderbird (and Firefox and Seamonkey) is the built in support for 3rd party addons. I occasionally try other email clients and always come back to Thunderbird precisely because it is so highly customizable.

Zenos it's "unknown" if I don't know what it is! And know nothing about it or it even exists.

A good customer service or sales representative or support staff will think about questions that might be asked.. and answer them before they are asked. Which is what I did while I was in the "Sales / Service" game. By doing this it saved a tremendous amount of time and $$$.. and created happy "customers" .. which leads to referrals!

All this rhetoric and I still don't know what an "add-on" is.. or how to find it.. or what it even does. 

I just wanted to clean up my Local Folders so I didn't have to contend with - Do you want to compress your files now?

Did you just learn anything?

But you know about it now. I can't help that you choose to do no research.

Typing thunderbird add-ons into your favourite search engine would have got you many articles in Thunderbird and its add-ons.

To cut to the chase, go to Tools|Add-ons|Extensions in Thunderbird.

Type ImportExportTools into the search box.

It will locate an add-on, located here: https://addons.mozilla.org/en-US/thunderbird/addon/importexporttools/

What have I learnt? That some users will not explore what is in front of them. All of your programs have a menu, and that menu exposes everything that your program can do. In this case Tools leads you to Add-ons.

Your choice, but your task will be easier with the aid of this add-on.

And you wouldn't have that "nag" warning message if you allowed Thunderbird to compact. Moving messages out of Thunderbird is not the best answer. So we have an XY problem too.

http://mywiki.wooledge.org/XyProblem

If you'd asked how to deal with the "Do you want to compress your files now?" question (actually, you meant "compact", not "compress") you'd have got to a more suitable solution quicker.

What was I supposed to anticipate when you asked about moving files and the real problem is a message about compacting?