Unlike Tab Session Manager, it does not peg the CPU on the thread running FF. It creates bookmarks for all open tabs, and like Tab Session Manager, it can save sessions on an automatic timer. It might seem that if you bookmark a page and it gets saved, then it will change the bookmark's location to one of the saved session bookmark folders. This is not what happens. Instead, FF has acquired the ability to store a bookmark in more than one folder. There is only one major drawback of doing it this way, so far as I've found in the few days I've been using it. The little blue star that indicates if a bookmark is already saved, and the Add Bookmark dialog, indicate that the tab is already saved in one of the session folders, as if it had been manually bookmarked. I think it would be much better if it were stored in the browser's database, like Tab Session Manager has converted to.