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 @
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{
.tabs-newtab-button > .toolbarbutton-icon{

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