Get Firebug console output using Selenium WebDriver

I want to use some values for my Selenium test. I can easily get this values via Firebug console

I was trying to do it using JavascriptExecutor:

 public void getSomeValue() {
    String command = "screenX"
    Object jsResult = ((JavascriptExecutor) driver).executeScript(command);
    System.out.println(jsResult.toString());
 }

But I've got java.lang.NullPointerException.

Can anybody explain me - why?

Thanks.