tcp/udp ports in the firewall on the ubuntu machines? and just a tcp exception on the Mac box in the firewall? Scan the (old) info here
http://forums.fireflymediaserver.org/viewtopic.php?t=4976
it’s relevant for the mac os box and a search under ubuntu and/or firewall may give you the relevant tcp/udp exceptions you need for the ubuntu ones. (and an upgrade to a more recent nightly may be of help..)