Not only is the new Tree Style Tabs a step backwards from the old, I'm also not running that addon, but this one: https://addons.mozilla.org/en-US/firefox/addon/tab-tree/ I find it lighter and more responsive.
As for the session manager, I need more features: saving and loading several sessions, etc.
I've always been satisfied with the built in session management (restore last session on open) and don't have any feedback on the other.