My primary complaint about Tree Style Tabs is that the nesting gets unwieldy. I have been meaning to check out Vertical Tabs (https://addons.mozilla.org/en-US/firefox/addon/vertical-tabs...). It's by the same developer who made BarTab. Some of the reasoning behind it is explained here http://philikon.wordpress.com/2010/05/01/tabs-tabs-tabs/. It similarly limits the tabs to a single category. I'll have to try out this Pano extension too.