Ironically, I set up an openfire server at home last night to use with the MSN and Yahoo transports I liked the idea of openfire because I'm a Java developer myself.
The one problem I did find, which is tablet related not openfire related, is that if I've got a proxy specified then my n800 won't connect to ANY jabber server (internal or external). This is a problem for me because I run a squid proxy at home (primarily to strip adverts).
The was a bug filed for this (https://bugs.maemo.org/show_bug.cgi?id=718) in 2006, which was reclassified as an enhancement (!!!) but nothing seems to have happened about it. I'm not sure how asking for the jabber client to work behind a system-wide proxy can be classed as an enhancement, but ...