I tried testing this extension, but have decided to keep using Firefox's auto-restore feature. Two reasons:
- Loading a large number of tabs and windows (>100 tabs at times) takes much longer than the built in feature. This extension will load all windows and tabs, but have them remain as "New tab" for far longer than I am comfortable with (somewhere between 30-60 seconds) Testing with 50 tabs takes about 15-20 seconds.
- I have a bunch of hidden (?) tabs from when I used Tab Groups, and in my trial, those tabs loaded in when I used the "Load with existing windows" feature for a manually saved session. From what I could tell, the extension simply made no distinction between normal and hidden tabs. Admittedly, there is nothing from the hidden tabs that couldn't be filed under bookmarks and forgotten, but it does seem to be a compatibility issue.
If these issues are fixed, then I will gladly use this extension to its fullest without the need for the session auto-restore.

Although I haven't used this extension for long, some good points are:
- UI is sufficiently fast and okay to use*.
- Works fine with session restore, though when I tried to close a few windows to clear out my session it seems the last few stayed. I'm not sure whether or not that's Firefox's fault, though.
- You seem to be putting some considerable effort into this extension and listening to feedback.
- It doesn't load in every single restored tab like Chrome extensions do...
- Tab and Window manipulation is extensive and easy enough to use for me
- Tooltips/popover/mouseover text for the buttons that weren't immediately clear.

* Honestly, I would prefer to have something like Session Buddy which I use for Chromium; the UI for it is intuitive and clean and quickly does what I need. Though I'm not very fair in my comparison; my use case for each browser is quite different.

Finally, some other improvements/enhancements I could see are:
- A cleaner and understandable icon
- Number of tabs open on the icon to shame me
- Persist the sort order for the backup pane in each view (those sessions/backups tabs) as well as the chosen view
- Make the font for the descriptions under each window name bigger; the text is aliased and hard to read.
- Ability to configure automatic backups
- Checkboxes for tab/window manipulation, perhaps? Make it easier to work with large numbers of tabs.
- Option to open the popup pane in a new tab. It's a bit cramped.
- Live tab/window manipulation! From what I can tell, this only operates on saved sessions.
- Auto-saving (not auto-backuping) the session each exit would be nice. Eventually I'll want to find something from an older session, and I need assurance that it'll always be there, like my history.

great addon. Kindly add option to save single, current active tab to save in session.

Works very well!
I would just like to see a few more features :
- Being able to disable or modify the auto-backup
- Add different categories for saved sessions
- Edit and export saved sessions

Good stuff so far. Just needs to automatically save the session upon closing the browser and automatically open the most recent session when opening the browser for 5 stars.

Edit: After using it for a little while now, I've run into a bug that I can reproduce easily. I use New Tab Tools. Open a new tab which goes to the New Tab Tools page, click on one of the pinned sites, click back to the tab next to it, go back to the new tab and it will randomly pick one of your saved tabs and reload it in this new one leaving you with two of the same tabs.