XAUTHORITY=
Xvfb :2 -screen 0 800x600x32 -fbdir /tmp &
DISPLAY=:2.0 firefox -width 800 -height 600 & 

XAUTHORITY=
DISPLAY=:2.0 firefox -remote "openURL(http://example.com)"
sleep 5 # give the page time to load
cp /tmp/Xvfb_screen0 /tmp/screenshot.xwd