The preference startup.homepage_welcome_url.additional causes problems with
official branded builds that has been known to break certain Marionette tests,
such as testing/marionette/harness/marionette/tests/unit/test_window_handles.py
and testing/marionette/harness/marionette/tests/unit/test_selected.py.

Many Marionette tests make assumptions about the number of open windows, and
this aligns the test environment of the officially branded builds to be more
similar to that of Nightly builds.