diff --git a/xpcom/components/nsComponentManager.cpp b/xpcom/components/nsComponentManager.cpp
index 05e5909c5d1..43f8058ef63 100644
--- a/xpcom/components/nsComponentManager.cpp
+++ b/xpcom/components/nsComponentManager.cpp
@@ -1102,9 +1102,11 @@ nsComponentManagerImpl::CollectReports(nsIHandleReportCallback* aHandleReport,
 }
 
 size_t nsComponentManagerImpl::SizeOfIncludingThis(
-    mozilla::MallocSizeOf aMallocSizeOf) const {
+    mozilla::MallocSizeOf aMallocSizeOf) {
   size_t n = aMallocSizeOf(this);
 
+  MonitorAutoLock lock(mLock);
+
   n += mFactories.ShallowSizeOfExcludingThis(aMallocSizeOf);
   for (const auto& data : mFactories.Values()) {
     n += data->SizeOfIncludingThis(aMallocSizeOf);
diff --git a/xpcom/components/nsComponentManager.h b/xpcom/components/nsComponentManager.h
index 174155b1967..0ef8f83a659 100644
--- a/xpcom/components/nsComponentManager.h
+++ b/xpcom/components/nsComponentManager.h
@@ -159,7 +159,7 @@ class nsComponentManagerImpl final : public nsIComponentManager,
 
   bool mJSLoaderReady = false;
 
-  size_t SizeOfIncludingThis(mozilla::MallocSizeOf aMallocSizeOf) const;
+  size_t SizeOfIncludingThis(mozilla::MallocSizeOf aMallocSizeOf);
 
  private:
   ~nsComponentManagerImpl();