here is the info in brief steps:

1. start thunderbird on old PC
2. click tools>export and then click 'open profile folder'
3. exit thunderbird
4. copy contents of the profile folder.
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 windows profile, overlaying contents of profile.
10. if there is a compatibilitie.ini file, delete it
11. restart thunderbird

Thank you David for your reply, but, I am confused though.

In step #2, 3 and 4 you say to in #2 to Open, #3 to exit then #4 to copy content profile folder. For me if I exit in #3 how can I copy the content if I just "exited" ? How can I copy if I exited? And the same with #7 to 9. You say to "exit" but where do I paste if I exited?

Thank you for your time and efforts to help me.

OK, I don't mean to confuse. If you go through the steps, you will discover that, - at the end of step 2, there are two tasks running: thunderbird and windows file explorer. when you exit thunderbir (step 3), you will be in windows file explorer, looking at the profile - select all and copy to external media (step 4) - the same happens at step 7. Step through it for an 'aha' moment. :) Let me know.