title: How to make sure the dimensions of screenshot taken in headless mode Firefox driver are always same?
body: <p>I want to take screenshot of a local html page in full-screen. But the following code produces different dimensional files(screenshots) for different html files. I find it strange as I am doing it full-screen, It should not happen. Please help.</p>
  
  <pre><code>firefox_options = Options()
  firefox_options.add_argument("--headless")
  driver = webdriver.Firefox(firefox_options=firefox_options)
  driver.get('file:///'+q.html.path)
  driver.fullscreen_window()
  shot = driver.get_screenshot_as_png()
  driver.close()
  </code></pre>