--- a/testing/marionette/harness/marionette/runner/base.py
+++ b/testing/marionette/harness/marionette/runner/base.py
@@ -247,17 +247,21 @@ class BaseMarionetteArguments(ArgumentPa
             if not os.access(path, os.F_OK):
                 os.makedirs(path)
             return path
 
         self.argument_containers = []
         self.add_argument('tests',
                           nargs='*',
                           default=[],
-                          help='Tests to run.')
+                          help='Tests to run. '
+                               'One or more paths to test files (Python or JS), '
+                               'manifest files (.ini) or directories. '
+                               'When a directory is specified, '
+                               'all test files in the directory will be run.')
         self.add_argument('-v', '--verbose',
                         action='count',
                         help='Increase verbosity to include debug messages with -v, '
                             'and trace messages with -vv.')
         self.add_argument('--address',
                         help='host:port of running Gecko instance to connect to')
         self.add_argument('--device',
                         dest='device_serial',
@@ -884,20 +888,25 @@ setReq.onerror = function() {
         return rv
 
     def add_test(self, test, expected='pass', test_container=None):
         filepath = os.path.abspath(test)
 
         if os.path.isdir(filepath):
             for root, dirs, files in os.walk(filepath):
                 for filename in files:
-                    if (filename.startswith('test_') and
+                    if (filename.endswith('.ini')):
+                        msg_tmpl = ("Ignoring manifest '{0}'; running all tests in '{1}'."
+                                    " See --help for details.")
+                        relpath = os.path.relpath(os.path.join(root, filename), filepath)
+                        self.logger.warning(msg_tmpl.format(relpath, filepath))
+                    elif (filename.startswith('test_') and
                         (filename.endswith('.py') or filename.endswith('.js'))):
-                        filepath = os.path.join(root, filename)
-                        self.add_test(filepath)
+                        test_file = os.path.join(root, filename)
+                        self.add_test(test_file)
             return
 
 
         file_ext = os.path.splitext(os.path.split(filepath)[-1])[1]
 
         if file_ext == '.ini':
             manifest = TestManifest()
             manifest.read(filepath)