The reason I asked that question twice is that the installer may well be a hidden (to you) dmg file that has been set to the default startup. It won't show in DU or Finder unless it is mounted, but the OS may well know about it and it may show in Startup Disc. If it's not there, just make sure that your internal drive IS the default boot drive. Once we know that, then we can go look for the installer. It's there somewhere because you see it when you Option-boot, but let's get your system booting normally and then we can find the culprit. So, a simple question that only needs a yes/no answer: Did you look at System Preferences/Startup Disc and set the default to your internal drive?
And can you say more about this Firefox partition? FF is an app, a browser, and doesn't need it's own partition, nor should it have a partition. In Post #11 you said there was no other partition showing in DU, so I'm confused by that.