Rated 4 out of 5 stars

You could do the same thing by editing your userChrome.css.

Here are some sites which will lead you through editing userChrome.css to hide the new tab button:
http://support.mozilla.com/tiki-view_forum_thread.php?locale=el&comments_parentId=372871&forumId=1
http://forums.mozillazine.org/viewtopic.php?f=38&t=1325445