With the deprecation of the Marionette client all consumers of it will also reach end of life in the near future. As such lets gets started and remove the firefox-ui harness and most of the tests, which are already covered by other harnesses.
Remaining tests can be quickly converted as Marionette tests, and included into the Mn jobs for now. Separate bugs will be filed for tests of each section in:
Removing the update tests can already be done and is covered by bug 1573406.