Firefox just opens with all the same tabs by default (at least it does for me. I don't recall if it's a setting I selected). It's a setting. In your preference

Firefox just opens with all the same tabs by default (at least it does for me. I don't recall if it's a setting I selected). It's a setting. In your preferences, under start-up, check restore previous session. IIRC, you can also restore the previous session from the history menu even if you don't have that option checked.