I suggest you use ImportExportTools (an add-on) to import your mail-folders

You can import every Mbox-file pref to a local folder

And use MoreFunctionsForAddressBook for Address-book import

Instructions these days leave out far too much. Like teaching one to drive a car. Push the gas pedal and the car will move. What about starting the car? What does this switch do? Where is the gas pedal? What does the other pedal do? Then CRASH!

Does there have to be a backup, copy, cvs file, text file of the email that is to be imported? There isn't one. Why are data files scattered all over the hard drive, in this folder and that folder? Why are there no consistent names for the files? What are those names? Does one import the EMT file, the BS file or the TXT file? Does the import have to be done from a backup or copy of the original emails? What if there is no backup?

The creators of these wonderful programs assume too much and you all know what assume does! The programmer knows what every switch and tag does in the program. The user does not know any of that. Most of the switches and tags could very well be left in the circular file for all the good they do.

Vague, incomplete and unintelligible instructions do nothing for the user and do less for the programmer. I do not want to nor do I have the time to rewrite the program to make it work. I do not want to have to add a bunch of switches to a command line to tell the program that I'm in the western hemisphere and it's dark outside. I don't need to guess what this or that will do. I want a straight forward means and method for getting the emails that are stored on one drive moved to another drive and available in one instance of TB.

What is a "profile" and what does it contain? Why does it even exist? Where are the actual emails stored? What is the name of the folder and file that emails are stored under and in?

Why does a user have to guess and guess again to try to get a simple task done? Maybe it isn't such a simple task and that is the fault of another group of programmers.

Programmers do not understand the real world and they can only communicate in hexadecimal. They don't even know why the standard for counting is to base 10.

This isn't programming.

Where did you see "EMT file, the BS file or the TXT file?" None of these are relevant to Thunderbird. Nor is much else of your rant.

Email messages are usually stored in a database or some other proprietary storage system. It isn't useful to put these into general circulation, because they are not appropriate for direct user access. You would break things if you changed them. Hence they are stored away from your normal Documents or Libraries folders.

https://support.mozilla.org/en-US/kb/profiles-tb

What is a "profile" and what does it contain? Why does it even exist? Where are the actual emails stored? What is the name of the folder and file that emails are stored under and in?

In Thunderbird's case, the chosen location for your email messages, address books, account settings is known as its profile. If you want to merge messages from one to another then you simply have to learn something about the profile.

Once you have found the old profile, you can use the tools mentioned above to import the relevant files to your current profile. You're being advised to use it to import mbox or mailbox files; there are other options and possibilities.

By default, your profile has a name containing a random string. So we can't tell you exactly what it is called, but we can tell you where to look.

Likewise you will likely have mail folders that you created and named, so we can't tell you what they are called either.

Microsoft tell us where to put such files, and by default, Microsoft make this location hidden.

But I can tell you that you'll be looking for files with no filename extension, such as Inbox or Sent. But if you have Microsoft's default settings, you won't be able to tell between them and their partner files, Inbox.msf, Sent.msf and so on, which you don't need. So you need to switch off two default settings, to reveal hidden files and to reveal filename extensions (stuff like .exe, .txt, .doc and so on.)

Are you getting the idea that at least some of your anger should be directed at Microsoft? ;-)

Whether or not you want to create backups or not is up to you. Backups are best practice before messing with stored data. But you don't need them.

"rewrite the program"? Where did that come from? Do you even know what "source code" means? Or what a compiler is? Changing settings is not "rewriting" a program! Command line switches? Where have they been mentioned?

It doesn't help to conflate all your general computer-related angst with one specific task.

There are 10 kinds of people. Those who understand binary and those who don't.

Instructions these days leave out far too much. Like teaching one to drive a car. Push the gas pedal and the car will move. What about starting the car? What does this switch do? Where is the gas pedal? What does the other pedal do? Then CRASH!

Actually, I am all in favour of a computer driving licence. Then people might be motivated to take lessons in the use of it, and so become able to make rational choices and avoid guessing.

You would learn little of the above from a manual or instruction book. Most of it you'd learn by example, under the guidance of an experienced and competent teacher. It's a shame people don't think computer use deserves similar effort.

I apologize, disregard my suggestions as it involve some settings. Programmers, you say, are not in the real world,( among inches, feet, miles, mm, cm, m, km, mile (!), old machine-inch, ......),I agree.