There's also Tab Center Redux, for people who already need a better way to handle tabs, but don't need or want to use trees there: https://addons.mozilla.org/en-US/firefox/addon/tab-center-re...

Thanks to Firefox's userChrome.css, you can pull pretty nice customized UIs even without any additional extensions :) https://github.com/eoger/tabcenter-redux/wiki/Custom-CSS-Twe...