The marionette_harness package contains the test runner for Marionette, and allows you to run automated tests written in Python for Gecko based applications. Therefore it offers the necessary testcase classes, which are based on the unittest framework.
For more information and the repository please checkout:
The following command will run the tests as specified via a manifest file, or test path, or test folder in Firefox:
marionette --binary %path_to_firefox% [manifest_file | test_file | test_folder]
To get an overview about all possible option run marionette --help.