If you know the url already then a locator like
`_fb_iframe_locator =  ('css selector', 'iframe[data-url="m.facebook.com/login.php"]')`

should suffice, then you can replace this whole sleep and loop with just:
```
fb_iframe = self.wait_for_element_present(*self._fb_iframe_locator)
self.marionette.switch_to_frame(fb_iframe)
```