The most useful and the most advanced session manager out there, across all browsers. The only feature that I initialy didn't find is the option for import end export of sessions. Only after having read in the description, that import/backup option is available from the Firefox integrated bookmark menu, I finaly found that option. My advice to the developer:
1. Remove the first tab from the left ("Active session", the one wih logo), as it only duplicates the content already present in the 2nd tab "Sessions", and also in the 3rd tab - "current session".
2. Then rename "Sessions" to "Saved sessions", and rename "History" to "Auto-saved sessions".
3. In the add-on "burger" menu, or as an extra button along all the other buttons, put an extra "Import/export" button, which would open a tooltip, explaining where to find (in the Firefox Bookmark Manager), and how to use the import/export function" (by simply importing/exporting Firefox bookmarks).
These 3 things will make the menu much more logical, user-friendly and easy to understand, although already now it's near perfection.
P.S. Chome's "Session Buddy" open as a new full-width tab on default, and thus it's easier to navigate and see all the sessions - yet another possible approach. Session Sync allows a maximum 800x600px.
P.S.S. "t's true that Active Session is duplicate with the one from 3rd tab - "Current Session" but the 2nd tab does not do that. " - What I meant, was that both in the list under the 1st tab, and in the same list under 2nd tab I see my saved sessions - i.e. the same saved session list under 2 different tabs. This creates confusion.
P.S.S.S - Don't want to seem annoying - just trying to hopefully help you improve this already a masterpiece.
"Since the active session can only be accessed through that button when the Auto-saved sessions is disabled (from configuration) I need to think really well how a good solution." Suggested solution: as mentioned above, remove the 1st tab, and rename the other two. Then, switch places of the remaining 2nd and 3rd tabs. Then the add-on tabs will look like this: 1st tab - "Auto-saved sessions", and the 2nd tab - "Saved sessions". The 1st tab should be open on default no matter whether auto-saving is switched on or off. If off, then the first entry in the list under the 1st tab shall be "current session (auto-saving switched off)" or "current session (to save, turn on autosaving)", or "current session (not saved; to save turn on auto-saving").
PS.x4: hopefully my last post :-) Yet another proposal - rename the addon to "Bookmark & Session manager" - this way not only users of session add-ons will find you, but also the much larger userbase of bookmark managers. Good luck!