Tree Style Tabs is essential if you keep many tabs open. Not only does it make it possible to read the window titles and use screen real estate more effectively, it facilitates tab hierachies (parent page A opened links to B, C, D, parent page B opened links to E, F, G, etc).
There is a very simple extension called Tab Counter which works very well together with TSTs and it helps to remind me when it's time to clean up the tab bar. It also appears to bring peace of mind that all tabs are still there, it's just the missing status bar. :)
Between Tree Style Tabs and The Fox Only Better[0] I can't see myself going to Chrome or any other browser. The UI is pretty much hidden until I need it, giving me maximum browsing space.
https://addons.mozilla.org/en-US/firefox/addon/tree-style-ta...
Though another comment has led me to wonder if FF doesn't have something similar built-in.