Every time I start up my computer, Firefox will automatically open. I close it at the end of each session, but no matter what Firefox will launch upon startup.
You have it ticked in your login items (Systenm Prefs/Accounts/Login Items), or else you accidently chose "open at login" from the menu that appears if you click on the dock icon.
Either untick it via the system prefs or disable it by going back to the dock pop-up menu.
