--- a/testing/marionette/mach_commands.py
+++ b/testing/marionette/mach_commands.py
@@ -146,29 +146,27 @@ class MachCommands(MachCommandBase):
     )
     def run_marionette_test(self, tests, **kwargs):
         if 'test_objects' in kwargs:
             tests = []
             for obj in kwargs['test_objects']:
                 tests.append(obj['file_relpath'])
             del kwargs['test_objects']
 
-        if conditions.is_firefox(self):
-            bin_path = self.get_binary_path('app')
-            if kwargs.get('binary') is not None:
-                print "Warning: ignoring '--binary' option, using binary at " + bin_path
-            kwargs['binary'] = bin_path
+        if not kwargs.get('binary') and conditions.is_firefox(self):
+            kwargs['binary'] = self.get_binary_path('app')
         return run_marionette(tests, topsrcdir=self.topsrcdir, **kwargs)
 
     @Command('session-test', category='testing',
         description='Run a Session test (Check Telemetry using marionette).',
         conditions=[conditions.is_firefox],
         parser=setup_session_argument_parser,
     )
     def run_session_test(self, tests, **kwargs):
         if 'test_objects' in kwargs:
             tests = []
             for obj in kwargs['test_objects']:
                 tests.append(obj['file_relpath'])
             del kwargs['test_objects']
 
-        kwargs['binary'] = self.get_binary_path('app')
-        return run_session(tests, topsrcdir=self.topsrcdir, **kwargs)
\ No newline at end of file
+        if not kwargs.get('binary') and conditions.is_firefox(self):
+            kwargs['binary'] = self.get_binary_path('app')
+        return run_session(tests, topsrcdir=self.topsrcdir, **kwargs)