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!

This is the only thing keeping me on firefox.

Their is simply no other browser that has tab management that comes anywhere close to firefox + sidebery.

The extension might be buggy from time to time when you have a lot of tabs and run the nightly FF version (I have around 3k), so turn on the snapshot feature to avoid loosing tabs :)

Perfect work! Practical functions, modern design, and abundant customization options leave me with nothing to fault.

I have no words for it. I have tried all other similar extensions, but no other offers the same tools and fast experience as this one. It's perfect and the best one for Firefox.
There is only one function missing, which is essential, being able to pause and play videos that are playing, there is only way to mute, but not pause.