diff --git a/toolkit/xre/nsXREDirProvider.cpp b/toolkit/xre/nsXREDirProvider.cpp
index 380c1c1..255539f 100644
--- a/toolkit/xre/nsXREDirProvider.cpp
+++ b/toolkit/xre/nsXREDirProvider.cpp
@@ -306,7 +306,8 @@ GetSystemParentDirectory(nsIFile** aFile)
                            "/usr/lib/mozilla"
 #endif
                            );
-  rv = NS_NewNativeLocalFile(dirname, false, getter_AddRefs(localDir));
+  const char* pathVar = PR_GetEnv("MOZ_SYSTEM_DIR");
+  rv = NS_NewNativeLocalFile((pathVar && *pathVar) ? nsDependentCString(pathVar) : reinterpret_cast<const nsCString&>(dirname), false, getter_AddRefs(localDir));
 #endif
 
   if (NS_SUCCEEDED(rv)) {