This is the ultimate tab management über-extension for Firefox. It is one of the two main reasons for using Firefox. (The other is Tridactyl.) It is so useful in managing a large number of tabs that I couldn't go without it, especially after I assigned keyboard shortcuts to its many different actions.

Sidebery is to Firefox what a critical medicine is to a pharmacy. Get it, learn it, and use it. It's a complete game-changer.