I can start the xvfb instance just fine
Xvfb :1 -screen 0 800x600x24
I can then launch firefox and have it load correctly but the window is always smaller than the resolution I set during the xvfb command.
firefox http://www.ebay.com --display=:1
You can see the black bar, that's the full resolution. It appears it's sizing to what the page wants but it will never fill it out completely.