Rated 3 out of 5 stars
Edit: somewhat buggy. Had to disable Tab Mix Plus which is somewhat of a deal-breaker (hoping this will be fixed) Creating a new tree is difficult (forces you to use a tab as a container instead of a "label" as a container. Dragging tabs around feels a lot clumsier than before. Dragging trees around is even worse. Clicking the container tab automatically expands the tree & no way to disable this behavior afaik (would prefer double click to expand) Thinking very strongly of using tab groups (Ctrl+Shift+E) instead of this addon. I'll try the addon a few days first.
Edit2: Alright, uninstalled, it's no good for me. Tab Mix Plus is an absolutely essential addon, and the fact that it is incompatible with this means I can't use this. Even if it were compatible, dragging tabs about and organizing tabs with this is just messy, and laggy too. I found another addon that does everything I wanted from this one perfectly: https://addons.mozilla.org/en-us/firefox/addon/tabgroups-manager/
Note: I'm using tabgroups manager with FF 20.0 apparently it's incompatible with newer versions (20.01 and v21+ etc.)