If your using the standard Firefox layout it will show icons of most visited sites but if you set a homepage those will not be present on the homepage url. You need to set bookmark if you have most visited sites so they stay saved for next time you use them.

Thanks, I do have those sites bookmarked so that's not the problem. Why do these unwanted sites appear as icons and how do I stop that? It's merely annoying.

Shortcut tiles will be auto-filled unless you edit them to a custom site.