Having an explicit session manager is paramount for working with multiple tabs and instances of Firefox. I had to re-install Windows and had literally no way of keeping my opened tabs and instances. I had to do it under the hood, by copying the contents of the last session, going through every file and making sure everything is synchronized. That's unacceptable.
Following the changes made post version 47... the disappearance of Tab Groups Manager, MAFF archiver and Session Manager were why I moved over to Google Chrome.. I would dearly love to resume using Firefox...