I forgot to doublecheck the port numbers in the capture log. Indeed, XMBC tries to connect on port 3689 whilst Firefly is listening on 9999.
Switching the Firefly portn number to 3689 and voila: works like a charm!
I tried all kinds of formatting options to enter the port number in the xbmc configuration file, but I can’t make it connect to another port then 3689. Probably someone who tried to implement a fix for i-Tunes 7 or so took some shortcuts and hardcoded a port number somewhere in the daap code.