How I wish there was a session manager for 57+. The Session Manager plugin was great but I'm not aware of any alternatives and Firefox doesn't seem to support a library of multiple past sessions to pick from.
Session Manager was one of the best extensions I relied on before Firefox 57. Since that project was discontinued due to deficiencies in WebExtensions APIs, I'd stopped updating Firefox on some machines (I also like Tab Mix Plus, which isn't yet released for Firefox 57+). On others, I recently installed Tab Session Manager. [1] It's not supposed to be a drop-in replacement for Session Manager, and I'm yet to use it well, but it does have basic session saving/restoring features.
