diff --git a/toolkit/xre/nsXREDirProvider.cpp b/toolkit/xre/nsXREDirProvider.cpp
index 6db876975187..5882c5d7f1d6 100644
--- a/toolkit/xre/nsXREDirProvider.cpp
+++ b/toolkit/xre/nsXREDirProvider.cpp
@@ -11,6 +11,7 @@
 
 #include "jsapi.h"
 #include "xpcpublic.h"
+#include "prenv.h"
 #include "prprf.h"
 
 #include "nsIAppStartup.h"
@@ -297,7 +297,8 @@ static nsresult GetSystemParentDirectory(nsIFile** aFile) {
       "/usr/lib/mozilla"_ns
 #    endif
       ;
-  rv = NS_NewNativeLocalFile(dirname, getter_AddRefs(localDir));
+  const char* pathVar = PR_GetEnv("MOZ_SYSTEM_DIR");
+  rv = NS_NewNativeLocalFile((pathVar && *pathVar) ? nsDependentCString(pathVar) : reinterpret_cast<const nsCString&>(dirname), getter_AddRefs(localDir));
 #  endif
 
   if (NS_SUCCEEDED(rv)) {