The simple things are often the best Rated 5 out of 5 stars

I always liked the close button on the tab bar instead of the tab. Now I can have both. I place it to the right of the URLbar and make it look like my stop button (different for everyone):
/* Close Button ext. - like URLbar stop-button */
@namespace url(http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul);
#cb-button image
{border:solid 1px #747f8a!important;
background-color:rgba(255,255,255,0.3)!important;
border-radius:2px!important;
padding:3px 2px!important;}
/* drop the '-moz-' for Fx16+ */
#cb-button:hover image
{background-image:-moz-linear-gradient(#FBAC96,#E5816E 49%,#D56658 51%,#C15244)!important;}

This review is for a previous version of the add-on (0.4.0.1-signed.1-signed).