some sites, like facebook, align centre by default - so there's not quite enough room at either side for a decent size window of something else. I'm sure the script is very basic (like 1 line)

Thanks

[path to profile folder]/chrome/userContent.css

@namespace url(http://www.w3.org/1999/xhtml);
@-moz-document domain(facebook.com) {
 #book { margin: 0 !important; }
}