Move away from Mozmill to Marionette for Tor Browser tests
Mozilla is not maintaining Mozmill anymore, in fact deprecating it. They are instead using Marionette. We should port our tests to Marionette as well and migrate to that framework in order to avoid weird Mozmill bugs which are not getting fixed anymore.
See: https://github.com/mozilla/firefox-ui-tests for the new test suite and
for some documentation.