Good start Rated 5 out of 5 stars

Works as advertised and I'm looking forward to the next version. All I really need now is a way to add custom images to the thumbnails.

Edit: Thanks to the author for providing an alternate solution by using this in userContent.css.

@-moz-document url("about:newtab") {
a[href="http://www.google.com/"] > span.newtab-thumbnail {
background-image: url("_url_of_new_image_") !important;
}
}

This review is for a previous version of the add-on (1.1-signed).