diff --git a/testing/firefox-ui/mach_commands.py b/testing/firefox-ui/mach_commands.py
index bfff0192337..75b6e54ffe6 100644
--- a/testing/firefox-ui/mach_commands.py
+++ b/testing/firefox-ui/mach_commands.py
@@ -60,5 +60,5 @@ class MachCommands(MachCommandBase):
              parser=setup_argument_parser,
              )
     def run_firefox_ui_test(self, tests, **kwargs):
-        kwargs['binary'] = self.get_binary_path('app')
+        kwargs['binary'] = kwargs['binary'] or self.get_binary_path('app')
         return run_firefox_ui_test(tests, topsrcdir=self.topsrcdir, **kwargs)