As bug 767315 has been shown we don't call endModule() for each of the test modules except the last one. Reason is that we call restart before we actually would reach the teardownModule call. https://github.com/mozilla/mozmill/blob/master/mozmill/mozmill/extension/resource/modules/frame.js#L716 As the comment states we don't want to do that but in this case we have to figure out a better solution. That means we should take the first modules start time and the last modules end time to calculate the duration of the test.
Mozmill will reach its end of life soon. We are currently working on getting all the tests for Firefox ported to Marionette. For status updates please see bug 1080766.