Okay, these steps may not be perfect but won't break anything. I am a novice with Linux, these are general guidelines:: One the Ubuntu machine

1. click help>troubleshootinginformation
2. scroll to 'profile folder' and click 'open folder'
3. exit thunderbird
4. you are now in profile, copy all of it to external media

On Linux Mint machine

1. install thunderbird, and start it
2. click help>troubleshootinginformation
3. scroll to 'profile folder' and click 'open folder'
4. exit thunderbird
5. you are now in profile. delete everything
6. paste contents of your saved profile from external media
7. restart thunderbird.
8. the ver 68 addressbook needs to be converted. click tools>import>importfromafile
9. select import addressbooks
10. select the Mork (mab) option
11. select the abook.mab file from within profile and click ok
12. repeat the previous 4 steps and select history.mab and click ok
13. check addressbooks to see if all is well.
14. at this point, I think it may work. there were other changes from ver 68 to 140, but I don't remember them. Let me know how it goes, as there are some Linux folks here who can correct my errors. I do not no explicit file locations that your Ubuntu system used, compared to Linux Mint. If some folders are not visible, then the locations may need to be adjusted in the Linux file manager. Good luck. :)

David, Thank you for reading and writing. Your instructions are very clear and I have been able to accomplish the export/copy of the old profile. I'm on the old machine here, the new one is in a different location so tomorrow I will follow part two of your instructions. I'll write again then.

Thank you.

--ajo

David! I do believe most everything is OK!. Your instructions worked. What I have tested so far works.

Some things such as the order of accounts in the left pane, the columns displayed in the right pane, did not copy over; I think I can figure out those items.

the function/status of "Unified Inboxes" did not copy over. I'll have to rebuild that.

Some other things I have yet to try and will get to later.

Overall I am satisfied. Thank you very much for your guidance.

Regards, ajo

Thanks for sharing that. It will help in assisting others. For the unified folders, I've found the quickest fix if it's not working is to exit thunderbird, locate the Mail folder in profile and DELETE 'smart mailboxes' and restart thunderbird. Order of accounts can be dragged up or down in the 'account settings' pane. I'm super pleased that all went ok. :)