Do you mean that you want buttons there to open that particular website to its home page, or that you want the buttons to do some kind of search when you click them?

You can add bookmarks to those sites on the Bookmarks Toolbar, but not directly to the main "Navigation" toolbar. For that you would need to use an add-on.

In case it's helpful: Bookmarks Toolbar - Display your favorite websites at the top of the Firefox window.

By the way, the checked icons should appear when you start typing in the search bar, so you can launch a search in one of those sites that way. This article has more info: Search with the Firefox Address Bar.