Great extension, though there's at least one bug and improvement I'd like to mention:

Bug:
When a saved session contains 2 windows, if the second window only contains an "about:newtab" tab, then clicking on it in the list opens the page within Tab Session Manager's pop-up window instead of the browser window.

Improvement:
I have hundreds of tabs in a single window. Browsing the list of all tabs per window could be more convenient if you could collapse the list of Window 1's tabs to quickly see the tabs in Window 2. This could be done by clicking Window 1's favicon. You could also include a setting that if a session contains more than one window, collapse all windows in the list by default. This would make it faster to get to a given window should you have multiple of them in a saved session.

Another suggestion would be to allow clicking on the window's name e.g. "Window 1" to open the entire window. This way you can restore just one of the windows in a saved session. Alternatively, "Window 1" could act as the collapse/expand functionality mentioned above, and an " - Open" link added to the right of it (this may be better UX design because the distance between the favicon and the "Window 1" word is quite small which could lead to errors such as accidentally opening a Window with hundreds of tabs).

Thanks!