Using 1.0.7 in Firefox 3.6, history is cloned even for fresh tabs: if I click on the New Tab button, the history of whatever tab I was viewing at the time will follow me to this new tab. Similarly, if I close all tabs, instead of being left with a completely-fresh tab, the history of the last tab viewed simply does not go away. Perhaps this was considered a feature, but to me it’s a bug. I just don’t see any use case where this is advantageous. It actually breaks my mental model of my browsing session.
Regarding that last feature/bug, it actually has its own bug: the cloned history does not contain the currently-viewed page but only the Back history. In other words, the current page in the history is replaced by about:blank.
I hope this issue is fixed. More importantly, I hope this extension is finally incorporated in Firefox proper soon!