This has been resolved. :)
It required editing the TDE Menu to select an icon from a specific
directory.
Reelcat (on PCLOS Forum) pointed me to
/usr/lib64/firefox/browser/chrome/icons/default and choose
default48.png. The Thunderbird icon to use is in
/usr/lib64/thunderbird/chrome/icons/default also choose default48.png.
Once the icons are changed, save the menu, close the editor, log out and
log back in.
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