This isn´t that hard, so here we go:

edit /etc/apt/sources.list to make the entries look like this one:

deb http://ftp.at.debian.org/debian/ lenny main contrib non-free

  
After that run the following commands:

aptitude update
aptitude install sun-java6-jre sun-java6-fonts

 Now download the Debian package from the openfire website and install it:

wget "http://www.igniterealtime.org/downloadServlet?filename=openfire/openfire_3.7.1_all.deb"
mv downloadServlet\?filename\=openfire%2Fopenfire_3.7.1_all.deb openfire_3.7.1_all.deb
dpkg -i openfire_3.7.1_all.deb

Enjoy!

Update: This solution has one downside: the start/stop script will give warnings on Lenny / Squeeze, you can fix that by editing the /etc/init.d/openfire file and add this lines just after #!/bin/bash:

### BEGIN INIT INFO
# Provides:          openfire
# Required-Start:    $remote_fs $syslog
# Required-Stop:     $remote_fs $syslog
# Default-Start:     2 3 4 5
# Default-Stop:      0 1 6
# Short-Description: Start daemon at boot time
# Description:       Enable service provided by daemon.
### END INIT INFO