Hey Tom, you can use following commands to launch Firefox browser on Mac using Selenium Webdriver:
import java.util.concurrent.TimeUnit;
import org.openqa.selenium.WebDriver;
import org.openqa.selenium.firefox.FirefoxDriver;
public class Facebooklogin{
   public static void main(String[] args) throws InterruptedException {
       WebDriver driver=new FirefoxDriver();
       driver.manage().window().maximize();
       driver.manage().timeouts().pageLoadTimeout(1, TimeUnit.SECONDS);
       driver.get("https://facebook.com/");
   }
}