This add-on is absolutely essential. In the absence of the proper API required for saving sessions, this is by far the best that can be done. It saves sessions as bookmarks, which means there is no need to export sessions, as they are conveniently stored in the bookmarks file, backed up regularly by Firefox internal engine. Managing the sessions is also more convenient, using Firefox's internal bookmark manager.

The only down side is that tabs' states are not recorded (filled form fields, containers and so on), but that's a minor trouble compared to losing all windows and tabs when Firefox's internal session manager fails (and it fails a lot!)