They only support client to server XMPP, not server to server XMPP.
Facebook Chat should be compatible with every XMPP client, but is not a full XMPP server.
So you can use any XMPP client for the Facebook Chat, but you can only chat with other Facebook users.
Which is a pity. Welcome to the 90s.
In my recent experience contacting friends who use GMX's xmpp service works pretty unreliable. Only friends with a gmx.net|gmx.com|gmx.de|gmx.at|gmx.ch address can be reached OK. They are routed via xmpp-gmx.gmx.net[126.96.36.199]. Friends with a gmx.biz|gmx.info|gmx.name|gmx.org|gmx.eu|gmx.tm|gmx.co.uk|gmx.ie|gmx.fr|gmx.lu|gmx.li|gmx.it|gmx.se|gmx.pt|gmx.es|...
For starters, strive for the simplest solution, try to check if it's not a problem in your pc:
Upgrade your OS to the latest version or set of packages
Do the same tests on another machine on the same network and if it's possible, another OS to discard OS or library problems.
The next thing I'll usually do is fire Wireshark and inspect the network ...
A Facebook account comes already with a Jabber ID, email@example.com and the password is the same as your account.
You can find it in the profile; it's called "username" or something like that.
However, a few months ago when you try to use and external XMPP client, Facebook will alert you to allow such type of connection. And now you can't use any ...
The best thing to do then is to hide them.
If you are using the mobile app, then touch and hold on that contact (if visible) and touch on hide contacts. Then if you want to view it, go to settings. There you will find all the hidden contacts.
AFAIK there isn't a way to prevent only the redundant notifications and keeping the initial notification for incoming messages.
What you could do is shut chat off in your Gmail window by going to Settings > Chat > Chat off
This should allow you to chat using an external product without the native Gmail windows opening up. The downside is that obviously if ...
Please verify that your entry in $HOME/.centerim/config matches this example.