I wanted to like this addon, since it's one of the few that doesn't use bookmarks to manage sessions, and it does well when restoring sessions. It doesn't seem to handle sessions with a lot of tabs very well, though - opening the session manager popup lags really bad and freezes the whole browser, and when I had this installed I noticed a periodic lag/stutter every so often that went away after disabling it (I'm guessing this was from a session being saved). I also noticed in recent release notes that the license has been changed to a closed source one. This is an absolute no-go for me, as I don't trust addons that aren't open source. As such, I'm permanently uninstalling this from my Firefox.