I really like this add-on because I can use it on FF57+ and it's compatible with Tree Style Tabs.
I have to say though currently I find myself rarely really using it besides for saving the one session that i have every few minutes.
Main reason for that is the restore behavior. Like someone else already mentioned, when you restore a session it opens a new window and loads every single tab of the session.
Loading as in actually opening and displaying each webpage.
It would be much better to have it open the tabs "unloaded" like when you initially start FF.

Next thing would be a few more options and functionalities.
- Prompt me for a session to load on start.
- Make sessions more like dynamic savestates, so i can overwrite them. E.g. I have a session 'General'. When I load that the manager should "know" that the session is loaded and later when the window is closed or the session saved it should save the changes that were made during the session was loaded into that same session state.
Right now you can only save a new session with same name but different timestamp.
- Give option to only save current window or a selection of windows to a specified session. So one can work with multiple sessions at the same time.
- Give option to load session to current window or open in new window / attach to current session or open as independently loaded session.
- use standard UI font
- introduce groups
- save and restore per tab history

With all of this it would be perfect (and basically a clone of legacy add-on "Session Manager")
I know it's a lot to ask and some things aren't even possible yet due to Mozilla not releasing the needed API.