I understand this fails because the url in the page is relative. Take a look at https://github.com/bobsilverberg/mozwebqa-test-templates/blob/updates/tests/test_home_page.py#L95 for an example of how to deal with this.  Also take a look at the entire test at https://github.com/bobsilverberg/mozwebqa-test-templates/blob/updates/tests/test_home_page.py#L81 for some additional ideas for tests like this.