Restart Thunderbird with add-ons disabled (Thunderbird Safe Mode). On the Help menu, click on "Restart with Add-ons Disabled". If Thunderbird works like normal, there is an Add-on or Theme interfering with normal operations. You will need to re-enable add-ons one at a time until you locate the offender.

Tried with add-ons disabled. Same problem...

Located an old bug which sounds very much the same as you mention. Please read the last comment number 25 as this resolved the issue. https://bugzilla.mozilla.org/show_bug.cgi?id=460278

I followed the steps outlined in the bugzilla article. It did fix the problem of opening emails BUT none of my old emails show up in the new installation. I can't afford to lose access to these emails. Is there a particular file that needs to be edited or replaced to fix this in my old profile?

TRy using an addon called 'ImportExporttools' to import the mbox files from the 'Local Folders' backup folder into the current 'Local Folders' mail account.

https://addons.mozilla.org/en-US/thunderbird/addon/importexporttools/

Toad-Hall... You solution worked. Lots of sorting of the email as the add-on dumped it all in the Inbox without the sub-folder file structure. Oh well. At this point, I call it a win to know I have all my old emails.

Thanks to Toad-Hall and Matt for your input on this problem.

Steps: Copy old profile from: C:\Users\<name>\AppData\Roaming\Thunderbird\Profiles\<codename>.default Uninstall Thunderbird and delete the Thunderbird directory/folder at the location where I copied from. Install freshly downloaded copy of Thunderbird. Install extension/add-on Toad-Hall recommended; 'ImportExporttools' Add all email accounts. Import email files via the new add-on; Tools--ImportExportTools--Import mbox files. Point at saved profile.