Save all opened tabs in Firefox

Historically some of us don't trust Firefox to store all opened tabs on close - Tab mix plus addon was one of mine favorites.. But from version_3_God's_know_what_exactly Firefox has native/default mechanism to save tabs. What is interesting- my PRO (developers and sys admins) friends/colleagues don't know how to activate Firefox tabs saver but other NORMAL people do know. This is for you, my lovely geeks:

1. Open Firefox -> Tools -> Options -> Main -> Startup
2. Set When Firefox starts: Show my windows and tabs from the last time