Two Ells » Setting the Default Mail Client for Firefox:
# Open about:config in your browser.
# Set the preference named “network.protocol-handler.external.mailto” to true; if it doesn’t exist, create it and set it to true.

ok, so I changed it to false.
and that, yes, seems to have the effect I wanted: when I click on an email (or, under File menu, select 'Send Link'), nothing happens. Outlook does not open.

whereas moments ago, when te above preference was still set to true by default, Outlook opened. which was a pain, since it has to load. and useless, since I don't actually use Outlook. (I think it is just a trial version on this computer that came with it -?- not sure)