This extension allowed me to finally come back to Firefox after using some forks of older versions.
Truly excellent in that ability to have different tab panels with vertical tree tabs in them. Each panel is a different category. So much easier to keep organized and track of the tabs.
The only thing I miss from Tab Kit 2 is being able to color a tree parent and all child tabs of that tree one color to easily see those relationships at a glance.

I'm taking off one star because I have yet to find a compatible session manager that retains the tree and group structure.
This does not allow saving sessions (the snapshot feature auto-deletes as new ones are created, EVEN manually initiated snapshots get auto-deleted).