I thought I would try this to replace Session Manager, which will likely not work with FF 57. I disabled SM and installed this, saving my session with the toolbar button. When I restarted FF, I only see one tab, which happens to be the first tab in my session. (It's also a pinned tab, if it matters.) The session displayed in the toolbar dropdown has all of my previous tabs, so I'm not sure what's wrong. I did disable the tree-related option in the settings. (I'm using FF 56 still.) Lot of potential here, though!

Update: When I tried loading a session several days later (including after a restart of FF), it did indeed work.