I've been running circles around the "jabber account setup" in assistant. I tried using jabber.org (isode) and jabber.de (openfire), but whatever I do I get thrown back to an Error page

"Firefox can't establish a connection to the server at"

Assistant definetly tries to connect to the server (LittleSnitch is giving me a hint) and the connections are unblocked. Even after turning LittleSnitch off completely it won't work. On the other hand, I can connect to the servers using Jitsi without any problems. I'd be really thankful for any help.