I am unclear on you situation. If you have a newer PC, you can transfer 100% from the old to the new with these steps:

1. start thunderbird on old PC
2. click tools>export and then click 'open profile folder'
3. exit thunderbird
4. copy profile content to external media
5. install thunderbird on new pc, downloaded from thunderbird.net
6. in receiving PC, click help>troubleshootinginformation
7. scroll to 'profile folder' and click 'open folder'
8. exit thunderbird
9. now, paste contents of exported profile, overlaying contents of profile.
10. if there is a compatibilitie.ini file, delete it
11. restart thunder

if The old PC was a POP account and you just want to copy some of the messages to a different PC, that is also possible. Please clarify you intent. Thanks

Thank you for the detailed explanation. Once I get the new PC set up, I can try that process. Then would the whole Thunderbird open under the transferred profile(s)? Your comment about 'external media' may involve using a thumb drive, not a CD, to export the profile? Why in steps 3 and 8, would I exit Thunderbird before I copy or paste the profile info?

Yes, the old PC is a POP account. Would I do things differently then? There are files like 'family' and specific organizations that have emails saved under them, and I would like to save the whole files if possible. So there is the question: once my profile is open in a new PC, will those files be available in the new profile? Or do I need to download them from the old PC?

Thank you for your help! Susan C.H.

- CD or thunbdrive, whichever works for you. THumbdrives are just simpler. You are to exit thunderbird to ensure that thunderbird has released control of all the files. Otherwise, you might find the copied profile is incomplete. Yes, as I state in step 8, exit thunderbird prior to pasting the new profile. As a visual reminder, what is happening is that, when you step 2 and step 8, you will have both Thunderbird open and Windows File Explorer open. By exiting thunderbird, all you will be left with is file explorer. In going through the steps, it becomes obvious. The profile copes everything, absolutely everything. When you restart on new PC after pasting in the profile, you should see everything there, all folders, all accounts, and the complete addressbook, plus all of your settings.

Update. I reread your original post. Did I answer all of your concerns? If not, let me know. You also posed a concern about your account being POP if you like to keep all messages on your PC. The other option is to set an account as IMAP if the desire is to be able to access the account and messages from other computers or from a phone as well as the PC. It all depends on what you want.