--- a/toolkit/xre/nsXREDirProvider.cpp        2019-02-28 21:00:14.157543388 +0100
+++ b/toolkit/xre/nsXREDirProvider.cpp   2019-02-28 21:01:28.731128320 +0100
@@ -302 +302,2 @@
-  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));