Emulate (not exactly though) mobile screen in Firefox + Headless mode + save screenshot (JS)

gistfile1.txt

const webdriver = require('selenium-webdriver-3');
const firefox = require('selenium-webdriver-3/firefox');
const Profile = firefox.Profile;
const { DriverService } = require('selenium-webdriver-3/remote');

const capabilities = webdriver.Capabilities.firefox();
capabilities.setLoggingPrefs({ performance: 'ALL' });

const binary = new firefox.Binary();
binary.addArguments('-headless');
binary.addArguments('-width=1000');
binary.addArguments('-height=950');
const options = new firefox.Options(capabilities);
const profile = new Profile();
options.setBinary(binary);
profile.setPreference('general.useragent.override', 'Mozilla/5.0 (Linux; Android 4.2.1; en-us; Nexus 5 Build/JOP40D) AppleWebKit/535.19 (KHTML, like Gecko) Chrome/18.0.1025.166 Mobile Safari/535.19');
profile.setPreference('layout.css.devPixelsPerPx', '2');
options.setProfile(profile);

var srv = new DriverService.Builder("C:/Webdrivers/geckodriver.exe").setLoopback(true).build();
let driver = firefox.Driver.createSession(options, srv);

try {
    driver.manage().window().setSize(360, 640);
    driver.get('http://www.yahoo.com');

    driver.sleep(3000);

    driver.takeScreenshot().then(
        function (image, err) {
            require('fs').writeFile('out.png', image, 'base64', function (err) {
                if (err) console.log(err);
            });
        }
    );
} finally {
    driver.quit();
}