Site Home extension considers the site’s domain name as its main page. If a user navigates to the site’s root directory, the toolbar button is disabled.

“In an ideal browser interface the button with a home should take the user to the main page of a site he is browsing. This button should be disabled if the user has come to the main page.”
— Artemy Lebedev, 2001-11-08, http://www.artlebedev.com/mandership/75/#06