This declaration
WebDriver driver = new FirefoxDriver();
always opens a new instance window of Firefox. It doesn\'t use the already opened
In Java, when you say new a new object is instantiated. For WebDriver, every new is a new browser window.
new
If you want the same browser to be used then use the same driver object.
driver
driver.get("URL PATH");
This will go to the new Url with the already open browser.