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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.

NOTE: I had tons of problems until I disabled Tree Style Tab, and the plugin does warn to disable other extensions that manipulate tabs.

I've tried more than a dozen tab extensions over more than a decade. This is the best by far. It's rock solid and includes automatic backups that you can browse through and restore. You can have anything from a simple list to a compex structure of panels, each containing tabs and groups of tabs nested together as deep as you like. The customization options are insane without ever leaving the GUI.

The best add-on!

Is great with few bugs that if fixed would make it a 5 star.
Bugs:
Tabs merge with other panels sometimes
Shortcuts don't work when sidebar is not displayed
One of my panels lost all tabs once
Moving group to new window causes tabs to fall out of group
Extra Features:
Make snapshots more usable allowing you to rename them, remove and add tabs, organize them, restore or take snapshot of a selection, etc.