Update: Wow, it's getting better and better!
Features needed:
- more compact list of sessions (see Session Manager)
- save the scroll position of tabs (needs an API change)
- restore the back/forward history of tabs (needs an API change)

Done:
- ability to import sessions from Session Manager (first feature to add, as users won't migrate if they lose their sessions)
- only reload the selected tab of each window (nice workaround to bug 1378647!)
- ability to save only a window instead of all of them (see MySessions for a great GUI for it, but all windows should be saved by default)

Thanks!