--- a/dom/media/test/external/mach_commands.py
+++ b/dom/media/test/external/mach_commands.py
@@ -61,10 +61,10 @@ def run_external_media_test(tests, testt
 @CommandProvider
 class MachCommands(MachCommandBase):
     @Command('external-media-tests', category='testing',
              description='Run Firefox external media tests.',
              conditions=[conditions.is_firefox],
              parser=setup_argument_parser,
              )
     def run_external_media_test(self, tests, **kwargs):
-        kwargs['binary'] = self.get_binary_path('app')
+        kwargs['binary'] = kwargs['binary'] or self.get_binary_path('app')
         return run_external_media_test(tests, topsrcdir=self.topsrcdir, **kwargs)