Thanks a lot, Scott - it gives me hope!
mailto: links from FF open in Outlook here though I don't use it as a mail client.
But, can't follow your advice instantanously because there ain't no MAILTO entry in that protocol section (unprivileged user). I'll have to repeat it as an admin...
Edit:
Wrong section - next one is correct:
I have to search the program and assign it to the protocol.
(Always trouble to look up features like system control in a different language ;) )