The icon is not displayed in the latest version of Mozilla Firefox. However, as Murtuza says, if you're using an old version, you can't change the icon. The icon is configured by the developers and they are the only ones who can change it. If you want, you can change the Firefox icon from your desktop. Right-click the launcher, click Properties and open the Shortcut tab. Press the button and use the Browse option to select your icon. Click OK and then Apply.