--- a/browser/components/dirprovider/DirectoryProvider.cpp
+++ b/browser/components/dirprovider/DirectoryProvider.cpp
@@ -243,19 +243,28 @@ DirectoryProvider::GetFiles(const char *
   if (!strcmp(aKey, NS_APP_SEARCH_DIR_LIST)) {
     nsCOMPtr<nsIProperties> dirSvc
       (do_GetService(NS_DIRECTORY_SERVICE_CONTRACTID));
     if (!dirSvc)
       return NS_ERROR_FAILURE;
 
     nsCOMArray<nsIFile> baseFiles;
 
+    /**
+     * We want to preserve the following order, since the search service loads
+     * engines in first-loaded-wins order.
+     *   - extension search plugin locations (prepended below using
+     *     NS_NewUnionEnumerator)
+     *   - distro search plugin locations
+     *   - user search plugin locations (profile)
+     *   - app search plugin location (shipped engines)
+     */
     AppendDistroSearchDirs(dirSvc, baseFiles);
+    AppendFileKey(NS_APP_USER_SEARCH_DIR, dirSvc, baseFiles);
     AppendFileKey(NS_APP_SEARCH_DIR, dirSvc, baseFiles);
-    AppendFileKey(NS_APP_USER_SEARCH_DIR, dirSvc, baseFiles);
 
     nsCOMPtr<nsISimpleEnumerator> baseEnum;
     rv = NS_NewArrayEnumerator(getter_AddRefs(baseEnum), baseFiles);
     if (NS_FAILED(rv))
       return rv;
 
     nsCOMPtr<nsISimpleEnumerator> list;
     rv = dirSvc->Get(XRE_EXTENSIONS_DIR_LIST,