It doesn't look like the Session Manager extension will be making the transition to Firefox Quantum.
Session Manager stored your sessions in your Firefox profile folder. Here are instructions for locating your profile folder.
The sessions are in a "sessions" folder inside your profile folder, in files ending in ".session".
This has only been tested in Firefox (version 52), so I can't make any guarantees that it'll work in any other browser. But then, this is all for Firefox users anyway, isn't it?