> sometimes I get to a dead end and want to navigate back to where I was, but have somehow lost the thread of how I got there, either b/c I was doing cmd + click and lost the tab and / or closed it or because it's now buried in the current tab's history.

When I'm researching a subject (like general research), trying a few different approaches for search queries and keywords until I get a result page with some promising results. Opening the promising results in background tabs, but at some point I just click, when I believe it's the last promising result, which then replaces the search result page and its history of previous queries. Then I try the opened tabs, quickly scanning them, closing some if they are obviously not what I need. At some point I decide I may need some more searches, but which of the tabs again had the history with the search queries in them? Did I close it maybe? It's often easier to coax Firefox's awesomebar into revealing one of my recent searches/history and continuing from there cause trying to dig up that one tab with the history you need, at best it only takes me completely out of my flow :)

One of the primary goals of this UX is to address that specific problem. It may not be the best UI for that specific task, but that's not only use case it supposed to serve. I would also argue it's far better UI than ones present in the current browsers.

If you have any specific suggestions how to improve UI to make it even better for that use case we'd be happy to get feedback.