Why we need IEDriver and ChromeDriver but not FirefoxDriver in Selenium WebDriver

-> This is because of "Native Browser approach" used in WebDriver.

-> Each and every browser have different JS Engine.

-> Selenium offers inbuilt driver based on Firefox JS Engine. So rest of browser (like IE,Chrome) requires respective drivers for each browser.

-> Chrome Driver have built based on Chrome driver JS Engine.

-> IE Driver have built based on IE driver JS Engine.

So like wise all browser need respective driver for run in selenium webdriver