Unfortunately do to limitations in the the provided APIs in Firefox 57, Session Manager cannot be implemented as a WebExtension add-on and will not work in Firefox 57 or higher. My only recommendation is to either not update to Firefox 57, use the Firefox ESR build or use a forked version of Firefox that does not remove support for add-ons such as CyberFox or Waterfox. Session Manager saves and restores the state of all or some windows - either when you want it or automatically at startup, after crashes or periodically.
It can also automatically save the state of open windows individually. The add-on is restartless and can be enabled and disabled on the fly. This extension replaces SessionSaver and Tab Mix Plus' session manager. Session Manager is compatible with Tab Mix Plus. It can import sessions from both SessionSaver and Tab Mix Plus. See for the complete users and options guide.
Session Manager Subsystem, or smss.exe, is a component of the Microsoft Windows NT family of operating systems, starting in Windows NT 3.1.It is executed during the startup process of those operating systems (it is the first user-mode process started by the kernel). Session Manager can save, backup, restore, and manage multiple Firefox tab sessions. It comes with the option of loading the last saved session, not loading a. Session Manager can save, backup, restore, and manage multiple Firefox tab sessions. It comes with the option of loading the last saved session, not loading a previous session, loading a specific older session, or asking for the user to choose. Already, that makes it more powerful than the native Firefox session manager.
Feedback can be sent to support email address listed to the top right. The later is preferred. If you are having problems, please open the Browser Console (Firefox/Tools -> Web Development -> Browser Console) and disable and re-enable Session Manager and look for errors in the error console. If there aren't any enable logging in the options menu and disabling and re-enable Session Manager again and send the logs to the email address listed on the right.
NOTE: I CANNOT OFFER SUPPORT THROUGH THE REVIEW PAGES, PLEASE USE THE SUPPORT LINKS TO THE RIGHT. IF REPORTING A PROBLEM, PLEASE READ THE ABOVE PARAGRAPH FOR INSTRUCTIONS. Note: Firefox 28 changes the way menus work.