I am the one who posted a request for help in the Thunderbird forum (Link: https://support.mozilla.org/en-US/questions/1282838 ). - note: I changed my profile name from mark2012 to mark2090.
To ensure this issue is clear as to what the problem is,
: Email file is external from the Thunderbird program, whether it was saved from Thunderbird to the desktop or from another email program (ie: Windows Mail APP).
: Opening the external EML file opens in Thunderbird, whether Thunderbird is the default program or opened in Thunderbird using windows "Open With" command.
: When opened in Thunderbird, any attempts to "Save as" in any of the formats offered by Thunderbird (EML, HTML, TXT, or .) fail, the save-as dialogue window does not appear.
: Same issue when trying to save-as "Template".
What I was trying to do is re-save the external EML file as an HTML file. The only way I could do that using Thunderbird was to open Thunderbird first, and then drag the external EML file into any one of the folders that exist in Thunderbird. Then the Save-as dialogue window works.
I see above that others have confirmed this issue exists in Thunderbird.