I can't live without Tree Style Tabs[0] in Firefox. It's The Thing that finally got me to switch back to FF and I will never look back.
0: https://addons.mozilla.org/en-US/firefox/addon/tree-style-ta...
0: https://addons.mozilla.org/en-US/firefox/addon/tree-style-ta...
Much nice I think, but had some bugs with sessions sometimes.