In short: you have copies of two Thunderbird profiles, one containing 32 GB of mails up to 2014-05-05, the other one with mails since then?

It seems that you are quite experienced. So continue to keep copies of those full profiles safe, separate and untouched. Even 32 GB should be no problem to keep somewhere on a disc, at least until your friends TB works perfectly again.

Personally I see two methods (you are right, IMAP would be nice for migration, except for the part of locally kept mails):

1. IMAP method:
Check if the provider of your friend offers IMAP (probably a 99 % chance, it's usually a matter of finding out the correct mail server settings, often they are even identical). No need to get a new mail address though; thus you keep the same credentials/authentication.
New TB profile ("other"): Create a new IMAP mail account in thunderbird. Important: leave the existing POP3 in Thunderbird untouched. Create a new IMAP (!) subfolder "migration" on that new IMAP Thunderbird account (or several folders if you like). Copy the new e-mails since 2014-05-05 to the newly created "migration" folder. Quit Thunderbird.
Current profile ("32 GB"): Basically you do the same here, i. e. create a new IMAP mail account in Thunderbird with the same e-mail address/server/auth settings etc. as above. You should now see your subfolder(s) and copy them to the existing POP3 Thunderbird account or wherever you like.
If you have tons of messages exceeding the provider limit, repeat this procedure (deleting mails in IMAP "migration" folder after copying).

2. Conventional copy method:
New TB profile ("other"): Stop the profile from getting mails (=> settings) or just go offline. Create a new folder in Thunderbird, usually this will be a subfolder of "Local Folders", name it e. g. "migration". Copy the new mails you need to transfer to this "migration" folder. Quit Thunderbird.
Using the file manager of the operation system (Windows? => Explorer) go to the profile folder (still new/"other" profile of course), similar to C:\Users\<Windows user name>\AppData\Roaming\Thunderbird\Profiles\<Profile name>\ => see http://kb.mozillazine.org/Profile_folder_-_Thunderbird, look for the folder Mail > Local Folders where you should find two files called "migration" and "migration.msf".
Copy those two files to the (computer with the) current profile (Thunderbird must be closed) (same subfolder location as source profile above), of course those files should not yet exist there. Shouldn't be too difficult, you seem to know already about profile folders on the local file system.
After starting Thunderbird with the current profile you should see the copied folder named "migration".

Your friend should now be able to use the current profile only. Don't delete the rest until everything works fine.

Thanks so much for the reply, and I apologize for not being clearer. This is a tangled mess I didn't explain well, and since I'm not familiar with how she has TB -- or her email accounts, for that matter -- set up, I did a poor job of explaining. Please allow me to try to clarify.

On her old computer, her complete TB profile was close to 32 GB. In her TB profile, there was a top level ImapMail folder, but IIRC, whatever files were in that were so small as to indicate to me they were empty or nearly so -- I don't know why that folder was there and neither did my friend.

Then, under the Mail folder, there were three sub-folders if my notes are correct: one called Local Folders (with well over 100 sub-folders and God only knows how many sub-sub, sub-sub-sub, and sub-sub-sub-sub folders), one for her email address, and another for (I'm guessing) a second email address. I'm not sure how those later two fit into the her email scheme and she didn't know, either. My guess -- and I could well be completely wrong -- is that email messages delivered to those two accounts are immediately "filtered" into Local Mail folders & sub-folders if they fit certain criteria and if not, they are left in an Inbox for her to manually deal with.

In theory, everything would have been fine had the "helpful" friend who copied the Profile to the external hard drive copied the whole profile. For reasons that completely baffle me, somehow at least some of the Local Folders sub-folders weren't copied to the external drive. That's not something I was aware of before placing those Profile files in the default TB profile folder on the new machine, and it's not something we discovered until after starting TB & it retrieved new messages.

So, let's call that Profile 1: it was incomplete but it retrieved new email on the new computer until she spotted the problem & we shut it down. As near as I can tell, she last accessed her email via TB on the old computer about 10 PM on 5/1, so when we started TB on the new computer around 2 PM on 5/4, it received emails for that time period.

To give you an idea of how incomplete the Local Folders was on Profile 1: on the old computer, it was 28.7 GB with 606 files (or folders -- my notes aren't clear). What the "helpful friend" copied was 23.8 GB with 160 files or folders. Yikes.

Then there's Profile 2, which is currently in use. Profile 2 is a complete copy of the Profile from the old computer plus new emails received after about 2 PM Sunday afternoon but minus new emails received by Profile 1 from about 10 PM 5/1 - about 2 PM on 5/4.

Does that help clarify? What I need to do is extract those "new" emails from Profile 1 to get them into Profile 2.

Perhaps I should also mention that Profile 1 doesn't actually exist anymore because when we got the complete profile off the old computer, I did as I did the first time: I removed the files & folders in the TB default profile and replaced them. They still exist on the computer, though, in a file folder I created on the Desktop.

In reading your response, I'm thinking you've clarified things for me, if you can please confirm that I'm on the right track.

Perhaps I can create Profile 3 with the files I folders I deleted that made Profile 1, and prevent it from receiving new emails. Once, I've done that, I can create the "migration" folder you suggest & move all the "new" emails from that time frame into the "migration" folder then exit TB. Next, I can just copy "migration" and "migration.mdf" into same sub-folder location in the current Profile 2 for my friend to deal with as she has time.

If I'm correctly understanding, that sounds a whole heck of a lot easier than my thinking -- in the reading I've done, I hadn't gleaned I could just simply copy something like "migration" and "migration.mdf" from one profile to another in TB! That's a very intuitive kind of solution, but since I'm not familiar with TB, I hadn't any hope that a solution would be so simple!

Well, it is a bit more complex than I thought. It probably does make sense to create "Profile 3". I understand you will have a way of accessing that "Profile 3" with Thunderbird (perhaps you fire up Thunderbird with Profile Manager http://kb.mozillazine.org/Profile_Manager ?), unless you have another way of telling what is missing by just looking at files/the file system (which I think is nearly impossible).

If I understand you correctly the difficult part is finding "new emails received by Profile 1 from about 10 PM 5/1 - about 2 PM on 5/4" or whatever delta you have (I don't really have to know), which isn't clearly separate in one way or another (like ideally, those delta mails could just be found in one folder or so, of course then it would be much easier). As you say, the problem started by not copying the complete profile.

So, assuming you create Profile 3 which contains the delta (or, cit. "those "new" emails from Profile 1 to get them into Profile 2") and you let Thunderbird access this profile, it seems to me that to find those e-mails is not easy, as they may be found in many folders and subfolders. But if those folders are arranged hierarchically, it may be a good idea to start a search on the top hierarchy of each folder to be considered: Left click on a folder > Search Messages > then perhaps selecting date as a search criteria (only days, no time possible), select "is before" or "is after" and by adding a second search criteria (click +) you could narrow down messages to be transferred. You could even "Save as Search Folder" (http://kb.mozillazine.org/Saved_Search) each search you'd have to make and copy the containing mails out of those search folders to a migration folder (search folders just show the mails found, but they remain in their original (sub)folders). Copying rather than moving increases the GB used but may be a bit safer.

If there aren't that many mails missing and not many folders and subfolders where they could possibly be, it's of course much easier to manually retrieve/copy mails by date/time out of the (sub)folders to a migration folder. Creating a (or several) migration folder(s) is just a suggestion since I believe it might be easiest to copy this/these migration folder(s) out of a profile to another one with a file manger, Thunderbird closed.

This may all sound a bit complicated, but you may want to be on the safe side ... these are all just suggestions from my personal experience, there may be other or better ways. Even if I didn't understand you 100 % you seem to have a good idea of what needs to be done, as far as I could tell. Well, good luck!

Many, many thanks! Your two responses have given me the info I needed in an understandable manner for someone who knows nothing about TB. I wasn't familiar with TB's Search & Saved Search functions, and I think those will be helpful.

You are correct that finding those messages is going to be quite tedious, but my hope is that once I've got my friend on the phone, she'll tell me that a lot of the folders & sub-folders are of an "archive" nature and don't receive new messages. If she's not sure, then perhaps looking at her filters might give me a better idea, and then I can focus on those locations. I hope.

For the sake of my sanity, I think I'll likely stick to only one Migration folder and then she can work on moving those emails to the appropriate location as she has time.

Many thanks for you guidance, and I hope you'll not see this thread again! I think we'll be attempting to do this tomorrow night & I'm hopeful that it will go smoothly even if tediously.

If ever you find some time you may briefly report back if and how you did it (possibly helping others later on). Good luck!