I've been using Tree Style Tabs[0] with Firefox for a long time. Being able to nest tabs and then collapse them is awesome. It is hard to live without it when using Chrome (I know there is a similar addon for Chrome[1], but it doesn't have the same feel since it just adds a new window off to the side and still has the tabs across the top).
[0] https://addons.mozilla.org/en-US/firefox/addon/tree-style-ta... [1] https://chrome.google.com/webstore/detail/sidewise-tree-styl...