I like it. Rated 5 out of 5 stars

Looks good (Firefox 9.0.1). Also coordinates well visually with this user style (for Stylish add-on; replaces ugly red tab close buttons and some others with attractive aqua-colored spheres):
http://userstyles.org/styles/2759/tab-close-button-mac-os-x-panther-style-aqua

This review is for a previous version of the add-on (1.0.1-signed). 

Thanks.