diff --git a/toolkit/xre/nsXREDirProvider.cpp b/toolkit/xre/nsXREDirProvider.cpp
index 64417770d971..7789a4aeac60 100644
--- a/toolkit/xre/nsXREDirProvider.cpp
+++ b/toolkit/xre/nsXREDirProvider.cpp
@@ -649,7 +649,10 @@ nsXREDirProvider::LoadExtensionBundleDirectories()
                                NS_EXTENSION_LOCATION);
       LoadExtensionDirectories(parser, "ThemeDirs", mThemeDirectories,
                                NS_SKIN_LOCATION);
-#ifdef MOZ_BUILD_APP_IS_BROWSER
+/* non-Firefox applications that use overrides in their default theme should
+ * define AC_DEFINE(MOZ_SEPARATE_MANIFEST_FOR_THEME_OVERRIDES) in their
+ * configure.in */
+#if defined(MOZ_BUILD_APP_IS_BROWSER) || defined(MOZ_SEPARATE_MANIFEST_FOR_THEME_OVERRIDES)
     } else {
       // In safe mode, still load the default theme directory:
       nsCOMPtr<nsIFile> themeManifest;