Downloaded the program on a laptop and did following steps:
File -> new libary ->
entered ip adress of the pc on which firefly media server is running and it worked on 2 machines.
Opened another server on a second laptop and i can reach it from every other pc (also with the original sever pc) with the java applet.
You said then it would be a problem woth my router not beeing able to process multicast properly.
I am running a Linksys WRT54GL Router with Tomato firmware in which i activated the multicast option. (see the attached picture)
Do you think it could be a problem with ubuntu and bonjour?
it really makes me mad!