I am absolutely proud of myself...
Just courageously searched my registry, fought and thought hard, and made changes without any backups whatsoever, and finally found an area to change that nasty bug.
Here it is:
change (Default) value Data to:
C:\Program Files\Netscape\Netscape\Netscp.exe -mail "%1"
When clicking on any future e-mails, navigator will no longer launch simutaneously with Composer.
NO RESTART NEEDED!!!