Here are the steps. This will bring a full copy of accounts and folders and settings to the new pc. The versions of thunderbird should be the same or nearly so. That is, if thunderbird on old PC is years behind on updates, I suggest updating there prior to copying to a pc with the latest release.

Here is the link for moving a profile in Windows when installed in default location:

https://support.mozilla.org/en-US/kb/moving-thunderbird-data-to-a-new-computer

here is the info in brief steps:

1. ensure thunderbird is NOT running on old machine
2. enter %appdata% in search window on task bar and press Enter key
3. this should open windows file explorer in Appdata\roaming folder
4. highlight the Thunderbird folder and copy to external media
5. ensure thunderbird is installed on second computer and NOT running
6. enter %appdata% in search window on task bar and press Enter key
7. you should be in Appdata\roaming folder , copy&paste the exported Thunderbird folder there. This may prompt to overwrite an existing folder of same name. allow it
8. start thunderbird and all should be there

OK will give it a try!! Thank you...will let you know. Question..I installed TB on new W11 but did NOT enter any email as requested right?

Yes, the profile copy will bring everything over.

Your instructions were great! The only issue I had was on the W11 when I put in the %appdata% is brought me to an internet search so I did a RUN and typed it there..all emails are there!!! Took a bit to update but they are there..getting used to this W11 is awful..menus are different and small etc. but I guess I will have to get used to that. Thank you again for your help.

Question to verify that these LOCAL folders are independent of each laptop and only the IMAP emails will be updated on either?

True, The local folders are independent. I'm glad all worked out for you. W11 is good, but takes some adjustment. :) Thanks for the feedback. :)

In looking t my profile in TB on W11 that I imported successfully, I now see three files..the bottom one on the screenshot attached (tx6) is the only one on the old W10 laptop. Should I delete the other two?

Thunderbird only uses one. The ones not used may be safely deleted. If unsure which is the active one, click help>troubleshooinginformation, scroll down to 'profile folder' and click 'open folder' You will be in winddows file explorer and the name of the profile folder should be there.

I checked via the help as you instructed and the bottom tx6 was the default. I deleted the other two and now it cannot find new email from yahoo..I reinstated them both and it find them (I sent myself a test). It seems the profile Qm50 is very small and if I delete that one only TB works but not if I delete the n6m profile.

This isn't adding up. Thunderbird uses only one profile. All I can guess is that the tx6 profile has a custom link to the other profile for that one account. You can verify that by clicking account settings and highlight the account pane and then click the 'server settings' pane and view bottom line. My guess is it's pointing to the other profile. Unusual.

I checked one and it is pointing to the tx6

Well, something is amiss, but if all is okay, I wouldn't worry yet. Still, at some point, you need to fix this, as transferring to a new computer may be problematic. I would check the server settings pane for all accounts and also the Local Folders pane. You can check those other profiles but clicking help>troubleshootinginformation, scroll down to 'profiles' and click 'about:profiles' and launch one of them. That will restart thunderbird with the other profile and you can view the contents. Then, repeat the steps to return to the default profile. When you had deleted the two profiles, what error message did you get?

OMG I will try on each of my emails ..lot of work. I did install /copy my profile as you instructed in the beginning of this question. If I delete the two I get a message that there is an Authentication error Here is another screenshot of profile ini..does that help?

I went thru all 8 email accounts and when I click on about: profiles they all say default is pointing to the tx6 for both email and local

Maybe if you post the exact error you receive when removing the other two profiles, we can resolve this. It is a real puzzle to me.

So, I MOVED the two odd profiles to a folder on my desktop and restarted W11. Then I sent an email from my phone to myself and opened in TB..and by magic it came thru. I have no idea why now it works . My profile now also shows the one file. This whole W11 conversion is just awful I am almost ready to give up but appreciate you help so much. I did notice in the properties of the profile there are two more listed..just left them alone. I think maybe I should back up this working profile but i forgot how?

Well, I respectfully disagree. You successfully coping the profile in your first attempt. Those extra profiles caused some confusion, but deleting wasn't necessary. Anyway, all is well. Here are steps to backup and possibly restore: here is the info in brief steps:

BACKUP

1. ensure thunderbird is not running
2. enter %appdata% in search window on task bar and press Enter key
3. this should open windows file explorer in Appdata\roaming folder
4. highlight the Thunderbird folder and copy to external media

RESTORE

1. ensure thunderbird is installed and NOT running
2. enter %appdata% in search window on task bar and press Enter key
3. you should be in Appdata\roaming folder , copy&paste the exported Thunderbird folder there. This may prompt to overwrite an existing folder of same name. allow it
4. start thunderbird and all should be there

David..I truly appreciate your help as I have limited knowledge and sometimes get lost with all the technical stuff..and that gets me frustrated ...your instructions were great and for now all is working so I will back up and leave well enough alone...again thank you.