You are facing this error because while initializing firefox driver, you have used wrong keyword for setting Property i.e. instead of using:
System.setProperty("Webdriver.gecko.driver","C:\\Users\\abby\\Desktop\\geckodriver.exe");
Use:
System.setProperty("webdriver.gecko.driver","C:\\Users\\abby\\Desktop\\geckodriver.exe");
webdriver.gecko.driver is the correct keyword for firefox driver. Now java is case-sensitive, so using Webdriver.gecko.driver, it will throw an error.
Hope this would help.