diff --git a/memory/build/mozjemalloc.cpp b/memory/build/mozjemalloc.cpp
index 58a92cd457ee..afcb663d182f 100644
--- a/memory/build/mozjemalloc.cpp
+++ b/memory/build/mozjemalloc.cpp
@@ -5216,6 +5216,9 @@ void _malloc_postfork_parent(void) MOZ_NO_THREAD_SAFETY_ANALYSIS {
 
 FORK_HOOK
 void _malloc_postfork_child(void) {
+  // Do this before iterating over the arenas.
+  gArenas.PostForkFixMainThread();
+
   // Reinitialize all mutexes, now that fork() has completed.
   huge_mtx.Init();
 
@@ -5225,7 +5228,6 @@ void _malloc_postfork_child(void) {
     arena->mLock.Reinit(gForkingThread);
   }
 
-  gArenas.PostForkFixMainThread();
   gArenas.mLock.Init();
 }
 #endif  // XP_WIN