title: browser.download.manager.showWhenStarting may not be overridden
body: <p>I'm using selenium webdriver for firefox on C# project. After I installed the latest webdriver (2.40) with NuGet Package manager this error occurred:</p>
  
  <blockquote>
    <p>The type initializer for 'Class' threw an exception. --->
    System.ArgumentException: Preference
    browser.download.manager.showWhenStarting may not be overridden:
    frozen value=False, requested value=False.</p>
  </blockquote>
  
  <p>My code:</p>
  
  <pre><code>FirefoxProfile profile = new FirefoxProfile();
  profile.SetPreference("browser.download.manager.showWhenStarting", false);
  webdriver = new FirefoxDriver(profile);
  </code></pre>
  
  <p>The error is raised on the second line.
  Do you have any idea how this can be solved?</p>