Mozilla will only open in a small window which won't enlarge and disappears when clicked
When the small window is clicked usually disappears but sometimes there is a shadow of a Page that slides off the screen to the right?
ప్రత్యుత్తరాలన్నీ (1)
You can check for problems caused by a corrupted localstore.rdf file.
You can try to open the system menu via Alt+Space and see if you can move the window into view and resize the window by using the keyboard.
Close and restart Firefox to see if you keep the setting.