Very Buggy Rated 2 out of 5 stars
I liked the idea of this add-on and found it useful and was using it for a while. Unfortunately, it's very buggy and has crashed Firefox.
Trying to organise your tabs by dragging and dropping is a nightmare. You drag a tab drop it somewhere and... nothing? It just hangs and you're left scratching you head, so you try again... again, nothing. You then have to reboot Firefox, open the Tree Style Tab sidebar again, and see if anything has happened - it's a very hit-and-miss affair. Pity, because it a good add-on if it worked. Trying to start a new group is even worst, a lot of messing about. And, don't even ask about changing the ordering of tabs, trying to switch a sub-tab so it becomes the main group tab is impossible.
I've uninstalled it and now just use the Firefox group functionality.
I'll have another look in the future, maybe it will get fixed.