Pass headless opts directly in at/firefox.test for #375 repro

at/firefox.test

@@ -5,7 +5,12 @@ use Selenium::Firefox;
 use Test::More tests => 2;
 use Test::Fatal;
 
-$ENV{MOZ_HEADLESS} = 1;
 my $driver;
-is( exception { $driver = Selenium::Firefox->new(); }, undef, "can spawn new Selenium::Firefox");
+is( exception { $driver = Selenium::Firefox->new(
+	extra_capabilities => {
+        'moz:firefoxOptions' => {
+            args    => [ '-headless' ],
+        },
+    },
+); }, undef, "can spawn new Selenium::Firefox");
 is( exception { $driver->shutdown_binary; }, undef, "can shutdown binary correctly");