Init.d to start/stop xvfb. Put it into /etc/init.d and chmod it to 755

console

apt-get install xvfb
apt-get install firefox

hudson steps

export DISPLAY=:99
/etc/init.d/xvfb start
# do your selenium tests here
/etc/init.d/xvfb stop

xvfb

XVFB=/usr/bin/Xvfb
XVFBARGS=":99 -screen 0 1024x768x24 -fbdir /var/run -ac"
PIDFILE=/var/run/xvfb.pid
case "$1" in
  start)
    echo -n "Starting virtual X frame buffer: Xvfb"
    start-stop-daemon --start --quiet --pidfile $PIDFILE --make-pidfile --background --exec $XVFB -- $XVFBARGS
    echo "."
    ;;
  stop)
    echo -n "Stopping virtual X frame buffer: Xvfb"
    start-stop-daemon --stop --quiet --pidfile $PIDFILE
    echo "."
    ;;
  restart)
    $0 stop
    $0 start
    ;;
  *)
        echo "Usage: /etc/init.d/xvfb {start|stop|restart}"
        exit 1
esac

exit 0