Thanks for the feedback. Session Boss is set out to work with saved session data and not on the live running tabs, so it doesn't make sense to close the current running tabs on the browser. For working with current running tabs, there's another add-on https://addons.mozilla.org/en-US/firefox/addon/tip-tab/, that can do all kinds of things to the current running tabs and windows, including closing them. Cheer.

Edit1 (to the 2nd response): A saved session is a snapshot of tabs and windows in time. The current state of the running tabs and windows could have been changed completely. There's really no relationship between the two. Even if text based tab navigation is preferred, it would be added to Tip Tab, not to this add-on.

Edit2 (to the 3rd response): You have a good point. Text-based manager is easier to work with lots of tabs. Icon-based is more flashy. Power users would rather use text-based with denser information overall. There's a TODO item to add tree hierarchy to tabs in Tip Tab, and it can be done in text. Also Tip Tab is still in development, admittedly not actively at the moment, and the next goal is to integrate with the bookmark system to bring the concepts of active windows/tabs and inactive windows/tabs with grouping, along the line of task-oriented or topic-oriented grouping of tabs, may be using the concept of containers or desktops. When I get more time, I'll get back to it.