How Selenium webdriver locate element with a http response full of JavaScript?

Question

I use selenium webdriver to speed up my testing. In my work our website will redirect to paypal for user to finish payment. However, I cannot make selenium webdriver to locate the email and password input field on paypal.

The sample paypal URL : https://www.sandbox.paypal.com/cgi-bin/webscr?cmd=_express-checkout&token=EC-07L974777B231831F#/checkout/login

A demo of my code may like this:

    WebDriver m_driver = new FirefoxDriver();
    String redirected_url = "https://www.sandbox.paypal.com/cgi-bin/webscr?cmd=_express-checkout&token=EC-0AT82163FM860854K#/checkout/login";
    
    m_driver.get(redirected_url);
    Thread.sleep(15*1000);
    WebElement we = m_driver.findElement(By.xpath(".//*[@id='email']"));
    we.sendKeys("login_email");
    we = m_driver.findElement(By.xpath(".//*[@id='password']"));
    we.sendKeys("login_password");
    we = m_driver.findElement(By.xpath(".//*[@id='btnLogin']"));
    we.click();

My problem:

With that code and on the paypal website, I always got an error message of 'no such element' found exception.

I can locate the element with firepath in Firefox but I cannot make selenium webdriver work.

I know this error may be caused by the JavaScript in the whole page of the paypal login page. I just don't know how to handle this situation.