Fantastic extension.

Vertical tabs in sidebar is so much better than unreadable horizontal tabs. F1 to open the vertical tabs. Only use Folder feature. You can open and close folders and sub-folders to keep the list manageable.

When you close Firefox and then reopen, your vertical tab list reappears. Perfect. No performance hit.

You can get rid of horizontal tab toolbar using userChrome.css. google it.

Dont know what Groups and Sessions are used for.

This extension makes Quantum worthwhile. But I still need a NoScript replacement.