diff --git a/js/xpconnect/loader/ScriptPreloader.cpp b/js/xpconnect/loader/ScriptPreloader.cpp
index f1efab936f56..86f9c4d513db 100644
--- a/js/xpconnect/loader/ScriptPreloader.cpp
+++ b/js/xpconnect/loader/ScriptPreloader.cpp
@@ -176,6 +176,7 @@ void ScriptPreloader::DeleteCacheDataSingleton() {
 
 void ScriptPreloader::InitContentChild(ContentParent& parent) {
   auto& cache = GetChildSingleton();
+  cache.mSaveMonitor.AssertOnWritingThread();
 
   // We want startup script data from the first process of a given type.
   // That process sends back its script data before it executes any
@@ -219,7 +220,7 @@ ProcessType ScriptPreloader::GetChildProcessType(const nsACString& remoteType) {
 ScriptPreloader::ScriptPreloader(AutoMemMap* cacheData)
     : mCacheData(cacheData),
       mMonitor("[ScriptPreloader.mMonitor]"),
-      mSaveMonitor("[ScriptPreloader.mSaveMonitor]") {
+      mSaveMonitor("[ScriptPreloader.mSaveMonitor]", this) {
   // We do not set the process type for child processes here because the
   // remoteType in ContentChild is not ready yet.
   if (XRE_IsParentProcess()) {
@@ -288,7 +289,7 @@ void ScriptPreloader::InvalidateCache() {
   }
 
   {
-    MonitorAutoLock saveMonitorAutoLock(mSaveMonitor);
+    MonitorSingleWriterAutoLock saveMonitorAutoLock(mSaveMonitor);
 
     mCacheInvalidated = true;
   }
@@ -428,6 +429,7 @@ Result<Ok, nsresult> ScriptPreloader::OpenCache() {
 // Opens the script cache file for this session, and initializes the script
 // cache based on its contents. See WriteCache for details of the cache file.
 Result<Ok, nsresult> ScriptPreloader::InitCache(const nsAString& basePath) {
+  mSaveMonitor.AssertOnWritingThread();
   mCacheInitialized = true;
   mBaseName = basePath;
 
@@ -453,6 +455,7 @@ Result<Ok, nsresult> ScriptPreloader::InitCache(const nsAString& basePath) {
 
 Result<Ok, nsresult> ScriptPreloader::InitCache(
     const Maybe<ipc::FileDescriptor>& cacheFile, ScriptCacheChild* cacheChild) {
+  mSaveMonitor.AssertOnWritingThread();
   MOZ_ASSERT(XRE_IsContentProcess());
 
   mCacheInitialized = true;
@@ -664,7 +667,7 @@ Result<Ok, nsresult> ScriptPreloader::WriteCache() {
   mSaveMonitor.AssertCurrentThreadOwns();
 
   if (!mDataPrepared && !mSaveComplete) {
-    MonitorAutoUnlock mau(mSaveMonitor);
+    MonitorSingleWriterAutoUnlock mau(mSaveMonitor);
 
     NS_DispatchToMainThread(
         NewRunnableMethod("ScriptPreloader::PrepareCacheWrite", this,
@@ -760,7 +763,7 @@ nsresult ScriptPreloader::GetName(nsACString& aName) {
 // Runs in the mSaveThread thread, and writes out the cache file for the next
 // session after a reasonable delay.
 nsresult ScriptPreloader::Run() {
-  MonitorAutoLock mal(mSaveMonitor);
+  MonitorSingleWriterAutoLock mal(mSaveMonitor);
 
   // Ideally wait about 10 seconds before saving, to avoid unnecessary IO
   // during early startup. But only if the cache hasn't been invalidated,
@@ -777,7 +780,7 @@ nsresult ScriptPreloader::Run() {
   Unused << NS_WARN_IF(result.isErr());
 
   {
-    MonitorAutoLock lock(mChildCache->mSaveMonitor);
+    MonitorSingleWriterAutoLock lock(mChildCache->mSaveMonitor);
     result = mChildCache->WriteCache();
   }
   Unused << NS_WARN_IF(result.isErr());
diff --git a/js/xpconnect/loader/ScriptPreloader.h b/js/xpconnect/loader/ScriptPreloader.h
index c41801953cd0..b98c0fd08020 100644
--- a/js/xpconnect/loader/ScriptPreloader.h
+++ b/js/xpconnect/loader/ScriptPreloader.h
@@ -71,7 +71,8 @@ class ScriptPreloader : public nsIObserver,
                         public nsIMemoryReporter,
                         public nsIRunnable,
                         public nsINamed,
-                        public nsIAsyncShutdownBlocker {
+                        public nsIAsyncShutdownBlocker,
+                        public SingleWriterLockOwner {
   MOZ_DEFINE_MALLOC_SIZE_OF(MallocSizeOf)
 
   friend class mozilla::loader::ScriptCacheChild;
@@ -104,6 +105,8 @@ class ScriptPreloader : public nsIObserver,
   static void FillCompileOptionsForCachedStencil(JS::CompileOptions& options);
   static void FillDecodeOptionsForCachedStencil(JS::DecodeOptions& options);
 
+  bool OnWritingThread() const override { return NS_IsMainThread(); }
+
   // Retrieves the stencil with the given cache key from the cache.
   // Returns null if the stencil is not cached.
   already_AddRefed<JS::Stencil> GetCachedStencil(
@@ -420,7 +423,7 @@ class ScriptPreloader : public nsIObserver,
   Result<Ok, nsresult> OpenCache();
 
   // Writes a new cache file to disk. Must not be called on the main thread.
-  Result<Ok, nsresult> WriteCache();
+  Result<Ok, nsresult> WriteCache() REQUIRES(mSaveMonitor);
 
   void StartCacheWrite();
 
@@ -485,7 +488,7 @@ class ScriptPreloader : public nsIObserver,
   bool mSaveComplete = false;
   bool mDataPrepared = false;
   // May only be changed on the main thread, while `mSaveMonitor` is held.
-  bool mCacheInvalidated = false;
+  bool mCacheInvalidated GUARDED_BY(mSaveMonitor) = false;
 
   // The list of scripts that we read from the initial startup cache file,
   // but have yet to initiate a decode task for.
@@ -501,11 +504,11 @@ class ScriptPreloader : public nsIObserver,
 
   // True if a runnable has been dispatched to the main thread to finish an
   // off-thread decode operation. Access only while 'mMonitor' is held.
-  bool mFinishDecodeRunnablePending = false;
+  bool mFinishDecodeRunnablePending GUARDED_BY(mMonitor) = false;
 
   // True is main-thread is blocked and we should notify with Monitor. Access
   // only while `mMonitor` is held.
-  bool mWaitingForDecode = false;
+  bool mWaitingForDecode GUARDED_BY(mMonitor) = false;
 
   // The process type of the current process.
   static ProcessType sProcessType;
@@ -530,8 +533,8 @@ class ScriptPreloader : public nsIObserver,
   // instance.
   AutoMemMap* mCacheData;
 
-  Monitor mMonitor MOZ_UNANNOTATED;
-  Monitor mSaveMonitor MOZ_UNANNOTATED;
+  Monitor mMonitor;
+  MonitorSingleWriter mSaveMonitor ACQUIRED_BEFORE(mMonitor);
 };
 
 }  // namespace mozilla