Can Selenium WebDriver open browser windows silently in the background?

There are a few ways, but it isn't a simple "set a configuration value". Unless you invest in a headless browser, which doesn't suit everyone's requirements, it is a little bit of a hack:

How to hide Firefox window (Selenium WebDriver)?

and

Is it possible to hide the browser in Selenium RC?

You can 'supposedly', pass in some parameters into Chrome, specifically: --no-startup-window

Note that for some browsers, especially Internet Explorer, it will hurt your tests to not have it run in focus.

You can also hack about a bit with AutoIt, to hide the window once it's opened.

If you are using Selenium web driver with Python, you can use PyVirtualDisplay, a Python wrapper for Xvfb and Xephyr.

PyVirtualDisplay needs Xvfb as a dependency. On Ubuntu, first install Xvfb:

sudo apt-get install xvfb

Then install PyVirtualDisplay from PyPI:

pip install pyvirtualdisplay

Sample Selenium script in Python in a headless mode with PyVirtualDisplay:

    #!/usr/bin/env python

    from pyvirtualdisplay import Display
    from selenium import webdriver

    display = Display(visible=0, size=(800, 600))
    display.start()

    # Now Firefox will run in a virtual display.
    # You will not see the browser.
    browser = webdriver.Firefox()
    browser.get('http://www.google.com')
    print browser.title
    browser.quit()

    display.stop()

EDIT

The initial answer was posted in 2014 and now we are at the cusp of 2018. Like everything else, browsers have also advanced. Chrome has a completely headless version now which eliminates the need to use any third-party libraries to hide the UI window. Sample code is as follows:

    from selenium import webdriver
    from selenium.webdriver.chrome.options import Options

    CHROME_PATH = '/usr/bin/google-chrome'
    CHROMEDRIVER_PATH = '/usr/bin/chromedriver'
    WINDOW_SIZE = "1920,1080"

    chrome_options = Options()
    chrome_options.add_argument("--headless")
    chrome_options.add_argument("--window-size=%s" % WINDOW_SIZE)
    chrome_options.binary_location = CHROME_PATH

    driver = webdriver.Chrome(executable_path=CHROMEDRIVER_PATH,
                              chrome_options=chrome_options
                             )
    driver.get("https://www.google.com")
    driver.get_screenshot_as_file("capture.png")
    driver.close()