What is your mozilla set to do when you re-open it, blank page, home page, or all tabs/windows open from last time? Try messing about with that open and restarting mozilla a few times

To me, it sounds like it's trying to remember stuff from last time but for some reason can't write your prefs correctly/at all when you close it, so it ends up opening the same thing as last time Does it happen if you create a new user profile? I could be wrong though