title: Set Firefox Preferences in Nightwatch
body: <p>How do you set firefox preferences in nightwatch? I would like to do the equivalent in java with nightwatch.</p>
  
  <pre><code>FirefoxProfile profile = new FirefoxProfile();
  profile.setPreference("intl.accept_languages", "de");
  WebDriver driver = new FirefoxDriver(profile);
  </code></pre>
  
  <p>I have this working in chrome, but again I can't figure out how to do it in Firefox.</p>
  
  <pre><code>"desiredCapabilities": {
    "browserName": "chrome",
    "javascriptEnabled": true,
    "acceptSslCerts": true,
    "chromeOptions" :{
      "prefs": {
        "intl.accept_languages":"fr"
      }
    }
  }
  </code></pre>
  
  <p>Thanks</p>