Make sure you’re using a recent SVN build of XBMC first. http://t3ch.yi.se/ for the latest builds.
Secondly, XBMC looks for only a specific port (forget what it is off hand, think it is 3869?), FF uses 9999 by default now iirc.
As a side note, XBMC is unable to see any music shared by iTunes 7.x, only iTunes 6.x shares will work with it.
But I can confirm that FF and XBMC do work together, as I tested it here at my GF’s on her softmodded XBOX.
When I’m back home I intend to softmod my own XBOX and get FF and XBMC working. When I do I’ll update the wiki appropriately as needed.