It's just CSS, lol Rated 3 out of 5 stars

Not to knock ya or anything, but this "Addon" is just some CSS code. Might want to keep this as a launching point into other CSS hacks you've concocted @ userstyles.org.
And oh, here's the CSS for those reading this... and yes, it DOES center. Just not in it's add-on form it seems.
.tabs-container > .tabbrowser-arrowscrollbox > .tabs-newtab-button{
-moz-box-flex:100;
-moz-box-pack:start;
width:auto;
}
.tabs-newtab-button > .toolbarbutton-icon{
padding-left:2px;
padding-right:2px;
}

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