[xahrepap]

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...

[itisame]

I can recommend alternatively https://addons.mozilla.org/en-US/firefox/addon/sidebery/

Much nice I think, but had some bugs with sessions sometimes.

[ColanR]

Second. It's the single biggest reason I'm still on firefox.