--- a/testing/firefox-ui/mach_commands.py
+++ b/testing/firefox-ui/mach_commands.py
@@ -55,10 +55,10 @@ def run_firefox_ui_test(tests, testtype=
 @CommandProvider
 class MachCommands(MachCommandBase):
     @Command('firefox-ui-test', category='testing',
              description='Run Firefox UI functional tests.',
              conditions=[conditions.is_firefox],
              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)