EDIT: It is now possible to customize the new tab page by adding your own CSS in the addon's preferences page from version 0.1.0 onwards.
The URL being shown is the result of a bug in the browser itself and was fixed about 13 days ago. Link - https://bugzilla.mozilla.org/show_bug.cgi?id=1372996
The URL should disappear whenever firefox sends an update containing this bug fix.
