firefox problem

Both my brother's macbook pro and my friend's macbook has this problem with firefox. Whenever they start their computer, the firefox DMG shows up and the "place firefox.app in applications" thing shows up. Even if I delete this DMG, it just shows up the next time they restart their computer.

Installing Leopard or the latest firefox update do nothing to solve this problem.

Did they put Firefox in the Applications folder yet? Just drag it from the window that opens into the Applications folder. It should stop.

Yes, it is allready installed in the applications folder. I even put the new one in the app folder replacing the old one to no avail.