This new layout actually breaks the tree tabs plugin. It relies on tabs-on-the-bottom (which are then pushed to the left), and the new layout drops support for it.

Of course, the authors could figure out a new way to do it, but for now it breaks the add-on.

God damn, this was one of the huge reason I was still sticking with Firefox...

It's still a few months out; I assume someone will find a workaround by then (if they haven't already).