By default when Firefox starts, it loads you home page. But you can change this if you want. Just get to the Preferences and go to the Main tab. The first entry in there is called When Firefox starts and it gives you three options to choose from. The first is the usual, Show my home page. But you can also choose to Show a blank page or (this is the one I like) Show my windows and tabs from last time. This last option basically restores Firefox to the state it was in before you closed it last time.