I hope you have a backup of your Thunderbird profile. How did you go about loading the profile in Ubuntu's installation of Thundebird? Sharing a Thunderbird profile between Windows and Linux is not something I would advice anyone to do, but if you won't take no for an answer, you've been forwarned and should therefore proceed knowing you are responsible for your data.

I did not do anything with TB profile, it comes installed with Ubuntu 20.04, I only set "Settings > Server Settings > Local Directory: to D:. You are the second person to "assume" I moved the profile and I don't understand why????

Assumptions are made when relevant and important information is left out, since we can't read minds. What is stored in D? Mbox files, whole profile folder?

"it will not load my emails from D:" only emails are stored on D:. Account Name

   Drafts
   Drafts.msf
   Inbox
   inbox.msf
   msgFilterRules.dat
   popstate.dat
   Sent
   Sent.msf
   Trash
   Trash.msf

That is everything, no files connected to TB are on D:.

You could try deleting those msf files while Tbird is NOT running, then relaunch it.

I have 2 of 4 laptops with Windows 10 that I use regularly and both have TB installed and accessing D: (they even access the backup USB that I store files on every night) without any problem, that makes me think that Ubuntu is reason TB is not working. I will try deleting .msf files after I back up. When I initially set up TB on Ubuntu, everything works, but when I reboot, then only the account names show up and nothing I do, including redoing Local Directory will get it working again.

At this point, I don't know a whole lot about Linux, but it appears to be ancient and incapable of doing a lot of things.