What's the value of `navigator.mozKeyboard` on > 1.3? If it's null, which a cursory search implies it might be, we can simplify this to:

```python
    # navigator.mozKeyboard is needed for v1.3 support
    self.marionette.execute_script("""
keyboard = navigator.mozKeyboard || navigator.mozInputMethod;
keyboard.removeFocus();""")
```