Edit footer menu item from Admin – Menus
Add the icon font awesome in the CSS CLASSES element.
You can find the CSS class for your icon on the Font Awesome homepage or by using the the cheatsheet. Each is prefixed by “fa-” to differentiate from any other common classes that may already be in use.
If you don’t see this element, Edit Screen Options from up right corner to enable it.
The CSS for the fontawesome icon may need a css to add spaces / font or something else. You can use this css to add custom elements.
And you replace “fa-comment” with your icon name.
The result from this example: