您仅将 实例声明为：
public WebDriver driver;
您还需要按以下方式对其进行初始化：
driver = new FirefoxDriver();
FirefoxOptions()有效代码块的实例将是：
System.setProperty("webdriver.firefox.driver","C:\\Users\\Nabila\\Downloads\\geckodriver-v0.26.0-win64\\geckodriver.exe");
FirefoxOptions options = new FirefoxOptions();
options.addArguments("test-type");
options.addArguments("start-maximized");
options.addArguments("--js-flags=--expose-gc");
options.addArguments("--enable-precise-memory-info");
options.addArguments("--disable-popup-blocking");
options.addArguments("--disable-default-apps");
options.addArguments("test-type=browser");
options.addArguments("disable-infobars");
driver = new FirefoxDriver(options);
driver.navigate().to("http://www.demo.guru99.com/V4/");
driver.manage().timeouts().implicitlyWait(300, TimeUnit.SECONDS);
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