diff --git a/dom/base/nsScriptLoader.cpp b/dom/base/nsScriptLoader.cpp
index b32d5b5131a7..88d7eefa6b3f 100644
--- a/dom/base/nsScriptLoader.cpp
+++ b/dom/base/nsScriptLoader.cpp
@@ -57,9 +57,11 @@
 #include "mozilla/dom/EncodingUtils.h"
 #include "mozilla/ConsoleReportCollector.h"
 
+#include "mozilla/AsyncEventDispatcher.h"
 #include "mozilla/Attributes.h"
 #include "mozilla/Unused.h"
 #include "nsIScriptError.h"
+#include "nsIOutputStream.h"
 
 using namespace mozilla;
 using namespace mozilla::dom;
@@ -102,11 +104,24 @@ ImplCycleCollectionTraverse(nsCycleCollectionTraversalCallback& aCallback,
 NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(nsScriptLoadRequest)
 NS_INTERFACE_MAP_END
 
-NS_IMPL_CYCLE_COLLECTION_0(nsScriptLoadRequest)
-
 NS_IMPL_CYCLE_COLLECTING_ADDREF(nsScriptLoadRequest)
 NS_IMPL_CYCLE_COLLECTING_RELEASE(nsScriptLoadRequest)
 
+NS_IMPL_CYCLE_COLLECTION_CLASS(nsScriptLoadRequest)
+
+NS_IMPL_CYCLE_COLLECTION_UNLINK_BEGIN(nsScriptLoadRequest)
+  NS_IMPL_CYCLE_COLLECTION_UNLINK(mCacheInfo)
+  tmp->DropBytecodeCacheReferences();
+NS_IMPL_CYCLE_COLLECTION_UNLINK_END
+
+NS_IMPL_CYCLE_COLLECTION_TRAVERSE_BEGIN(nsScriptLoadRequest)
+  NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mCacheInfo)
+NS_IMPL_CYCLE_COLLECTION_TRAVERSE_END
+
+NS_IMPL_CYCLE_COLLECTION_TRACE_BEGIN(nsScriptLoadRequest)
+  NS_IMPL_CYCLE_COLLECTION_TRACE_JS_MEMBER_CALLBACK(mScript)
+NS_IMPL_CYCLE_COLLECTION_TRACE_END
+
 nsScriptLoadRequest::~nsScriptLoadRequest()
 {
   // We should always clean up any off-thread script parsing resources.
@@ -115,6 +130,10 @@ nsScriptLoadRequest::~nsScriptLoadRequest()
   // But play it safe in release builds and try to clean them up here
   // as a fail safe.
   MaybeCancelOffThreadScript();
+
+  if (mScript) {
+    DropBytecodeCacheReferences();
+  }
 }
 
 void
@@ -543,6 +562,7 @@ nsScriptLoader::nsScriptLoader(nsIDocument *aDocument)
     mDeferEnabled(false),
     mDocumentParsingDone(false),
     mBlockingDOMContentLoaded(false),
+    mLoadEventFired(false),
     mReporter(new ConsoleReportCollector())
 {
 }
@@ -2162,9 +2182,9 @@ nsScriptLoader::ProcessRequest(nsScriptLoadRequest* aRequest)
     aRequest->MaybeCancelOffThreadScript();
   }
 
-  // Free any source data.
+  // Free any source data, but keep the bytecode content as we might have to
+  // save it later.
   aRequest->mScriptText.clearAndFree();
-  aRequest->mScriptBytecode.clearAndFree();
 
   return rv;
 }
@@ -2348,6 +2368,7 @@ nsScriptLoader::EvaluateScript(nsScriptLoadRequest* aRequest)
         MOZ_ASSERT(module);
         rv = nsJSUtils::ModuleEvaluation(aes.cx(), module);
       }
+      aRequest->mCacheInfo = nullptr;
     } else {
       JS::CompileOptions options(aes.cx());
       rv = FillCompileOptionsForRequest(aes, aRequest, global, &options);
@@ -2363,41 +2384,249 @@ nsScriptLoader::EvaluateScript(nsScriptLoadRequest* aRequest)
             rv = exec.DecodeAndExec(options, aRequest->mScriptBytecode,
                                     aRequest->mBytecodeOffset);
           }
+          // We do not expect to be saving anything when we already have some
+          // bytecode.
+          MOZ_ASSERT(!aRequest->mCacheInfo);
         } else {
-          nsJSUtils::ExecutionContext exec(aes.cx(), global);
           MOZ_ASSERT(aRequest->IsSource());
           if (aRequest->mOffThreadToken) {
             // Off-main-thread parsing.
             LOG(("ScriptLoadRequest (%p): Join (off-thread parsing) and Execute",
                  aRequest));
-            JS::Rooted<JSScript*> script(aes.cx());
-            rv = exec.JoinAndExec(&aRequest->mOffThreadToken, &script);
+            {
+              nsJSUtils::ExecutionContext exec(aes.cx(), global);
+              JS::Rooted<JSScript*> script(aes.cx());
+              if (!aRequest->mCacheInfo) {
+                rv = exec.JoinAndExec(&aRequest->mOffThreadToken, &script);
+                LOG(("ScriptLoadRequest (%p): Cannot cache anything (cacheInfo = nullptr)",
+                     aRequest));
+              } else {
+                MOZ_ASSERT(aRequest->mBytecodeOffset ==
+                           aRequest->mScriptBytecode.length());
+                rv = exec.JoinEncodeAndExec(&aRequest->mOffThreadToken,
+                                            aRequest->mScriptBytecode,
+                                            &script);
+                // Queue the current script load request to later save the bytecode.
+                if (NS_SUCCEEDED(rv)) {
+                  aRequest->mScript = script;
+                  HoldJSObjects(aRequest);
+                  RegisterForBytecodeEncoding(aRequest);
+                } else {
+                  LOG(("ScriptLoadRequest (%p): Cannot cache anything (rv = %X, script = %p, cacheInfo = %p)",
+                       aRequest, unsigned(rv), script.get(), aRequest->mCacheInfo.get()));
+                  aRequest->mCacheInfo = nullptr;
+                }
+              }
+            }
           } else {
             // Main thread parsing (inline and small scripts)
             LOG(("ScriptLoadRequest (%p): Compile And Exec", aRequest));
+            nsJSUtils::ExecutionContext exec(aes.cx(), global);
             nsAutoString inlineData;
             SourceBufferHolder srcBuf = GetScriptSource(aRequest, inlineData);
             rv = exec.CompileAndExec(options, srcBuf);
+            aRequest->mCacheInfo = nullptr;
           }
         }
 
       }
     }
+
+    // Even if we are not saving the bytecode of the current script, we have
+    // to trigger the encoding of the bytecode, as the current script can
+    // call functions of a script for which we are recording the bytecode.
+    MaybeTriggerBytecodeEncoding();
   }
 
   context->SetProcessingScriptTag(oldProcessingScriptTag);
   return rv;
 }
 
+void
+nsScriptLoader::RegisterForBytecodeEncoding(nsScriptLoadRequest* aRequest)
+{
+  MOZ_ASSERT(aRequest->mCacheInfo);
+  MOZ_ASSERT(aRequest->mScript);
+  mBytecodeEncodingQueue.AppendElement(aRequest);
+}
+
+void
+nsScriptLoader::LoadEventFired()
+{
+  mLoadEventFired = true;
+  MaybeTriggerBytecodeEncoding();
+}
+
+void
+nsScriptLoader::MaybeTriggerBytecodeEncoding()
+{
+  // We wait for the load event to be fired before saving the bytecode of
+  // any script to the cache. It is quite common to have load event
+  // listeners trigger more JavaScript execution, that we want to save as
+  // part of this start-up bytecode cache.
+  if (!mLoadEventFired) {
+    return;
+  }
+
+  // No need to fire any event if there is no bytecode to be saved.
+  if (mBytecodeEncodingQueue.isEmpty()) {
+    return;
+  }
+
+  // Wait until all scripts are loaded before saving the bytecode, such that
+  // we capture most of the intialization of the page.
+  if (HasPendingRequests()) {
+    return;
+  }
+
+  // Create a new runnable dedicated to encoding the content of the bytecode
+  // of all enqueued scripts. In case of failure, we give-up on encoding the
+  // bytecode.
+  nsCOMPtr<nsIRunnable> encoder =
+    NewRunnableMethod("nsScriptLoader::EncodeBytecode",
+                      this, &nsScriptLoader::EncodeBytecode);
+  if (NS_FAILED(NS_DispatchToCurrentThread(encoder))) {
+    GiveUpBytecodeEncoding();
+  }
+}
+
+void
+nsScriptLoader::EncodeBytecode()
+{
+  // If any script got added in the previous loop cycle, wait until all
+  // remaining script executions are completed, such that we capture most of
+  // the initialization.
+  if (HasPendingRequests()) {
+    return;
+  }
+
+  nsCOMPtr<nsIScriptGlobalObject> globalObject = GetScriptGlobalObject();
+  if (!globalObject) {
+    GiveUpBytecodeEncoding();
+    return;
+  }
+
+  nsCOMPtr<nsIScriptContext> context = globalObject->GetScriptContext();
+  if (!context) {
+    GiveUpBytecodeEncoding();
+    return;
+  }
+
+  AutoEntryScript aes(globalObject, "encode bytecode", true);
+  RefPtr<nsScriptLoadRequest> request;
+  while (!mBytecodeEncodingQueue.isEmpty()) {
+    request = mBytecodeEncodingQueue.StealFirst();
+    EncodeRequestBytecode(aes.cx(), request);
+    request->mScriptBytecode.clearAndFree();
+    request->DropBytecodeCacheReferences();
+  }
+}
+
+void
+nsScriptLoader::EncodeRequestBytecode(JSContext* aCx, nsScriptLoadRequest* aRequest)
+{
+  nsresult rv = NS_OK;
+  MOZ_ASSERT(aRequest->mCacheInfo);
+
+  JS::RootedScript script(aCx, aRequest->mScript);
+  if (!JS::FinishIncrementalEncoding(aCx, script)) {
+    LOG(("ScriptLoadRequest (%p): Cannot serialize bytecode",
+         aRequest));
+    return;
+  }
+
+  if (aRequest->mScriptBytecode.length() >= UINT32_MAX) {
+    LOG(("ScriptLoadRequest (%p): Bytecode cache is too large to be decoded correctly.",
+         aRequest));
+    return;
+  }
+
+  // Open the output stream to the cache entry alternate data storage. This
+  // might fail if the stream is already open by another request, in which
+  // case, we just ignore the current one.
+  nsCOMPtr<nsIOutputStream> output;
+  rv = aRequest->mCacheInfo->OpenAlternativeOutputStream(kBytecodeMimeType,
+                                                         getter_AddRefs(output));
+  if (NS_FAILED(rv)) {
+    LOG(("ScriptLoadRequest (%p): Cannot open bytecode cache (rv = %X, output = %p)",
+         aRequest, unsigned(rv), output.get()));
+    return;
+  }
+  MOZ_ASSERT(output);
+  auto closeOutStream = mozilla::MakeScopeExit([&]() {
+    nsresult rv = output->Close();
+    LOG(("ScriptLoadRequest (%p): Closing (rv = %X)",
+         aRequest, unsigned(rv)));
+  });
+
+  uint32_t n;
+  rv = output->Write(reinterpret_cast<char*>(aRequest->mScriptBytecode.begin()),
+                     aRequest->mScriptBytecode.length(), &n);
+  LOG(("ScriptLoadRequest (%p): Write bytecode cache (rv = %X, length = %u, written = %u)",
+       aRequest, unsigned(rv), unsigned(aRequest->mScriptBytecode.length()), n));
+  if (NS_FAILED(rv)) {
+    return;
+  }
+}
+
+void
+nsScriptLoadRequest::DropBytecodeCacheReferences()
+{
+  mCacheInfo = nullptr;
+  mScript = nullptr;
+  DropJSObjects(this);
+}
+
+void
+nsScriptLoader::GiveUpBytecodeEncoding()
+{
+  // Ideally we prefer to properly end the incremental encoder, such that we
+  // would not keep a large buffer around.  If we cannot, we fallback on the
+  // removal of all request from the current list.
+  nsCOMPtr<nsIScriptGlobalObject> globalObject = GetScriptGlobalObject();
+  if (globalObject) {
+    nsCOMPtr<nsIScriptContext> context = globalObject->GetScriptContext();
+    if (context) {
+      AutoEntryScript aes(globalObject, "give-up bytecode encoding", true);
+      JS::RootedScript script(aes.cx());
+      while (!mBytecodeEncodingQueue.isEmpty()) {
+        RefPtr<nsScriptLoadRequest> request = mBytecodeEncodingQueue.StealFirst();
+        LOG(("ScriptLoadRequest (%p): Cannot serialize bytecode", request.get()));
+        script.set(request->mScript);
+        Unused << JS::FinishIncrementalEncoding(aes.cx(), script);
+        request->mScriptBytecode.clearAndFree();
+        request->DropBytecodeCacheReferences();
+      }
+      return;
+    }
+  }
+
+  while (!mBytecodeEncodingQueue.isEmpty()) {
+    RefPtr<nsScriptLoadRequest> request = mBytecodeEncodingQueue.StealFirst();
+    LOG(("ScriptLoadRequest (%p): Cannot serialize bytecode", request.get()));
+    // Note: Do not clear the mScriptBytecode buffer, because the incremental
+    // encoder owned by the ScriptSource object still has a reference to this
+    // buffer. This reference would be removed as soon as the ScriptSource
+    // object would be GC.
+    request->mCacheInfo = nullptr;
+  }
+}
+
+bool
+nsScriptLoader::HasPendingRequests()
+{
+  return mParserBlockingRequest ||
+         !mXSLTRequests.isEmpty() ||
+         !mLoadedAsyncRequests.isEmpty() ||
+         !mNonAsyncExternalScriptInsertedRequests.isEmpty() ||
+         !mDeferRequests.isEmpty() ||
+         !mPendingChildLoaders.IsEmpty();
+}
+
 void
 nsScriptLoader::ProcessPendingRequestsAsync()
 {
-  if (mParserBlockingRequest ||
-      !mXSLTRequests.isEmpty() ||
-      !mLoadedAsyncRequests.isEmpty() ||
-      !mNonAsyncExternalScriptInsertedRequests.isEmpty() ||
-      !mDeferRequests.isEmpty() ||
-      !mPendingChildLoaders.IsEmpty()) {
+  if (HasPendingRequests()) {
     nsCOMPtr<nsIRunnable> task = NewRunnableMethod(this,
                                                    &nsScriptLoader::ProcessPendingRequests);
     if (mDocument) {
@@ -3425,9 +3654,24 @@ nsScriptLoadHandler::OnStreamComplete(nsIIncrementalStreamLoader* aLoader,
     }
   }
 
+  // Everything went well, keep the CacheInfoChannel alive such that we can
+  // later save the bytecode on the cache entry.
+  if (NS_SUCCEEDED(rv) && mRequest->IsSource() && IsBytecodeCacheEnabled()) {
+    mRequest->mCacheInfo = do_QueryInterface(channelRequest);
+    LOG(("ScriptLoadRequest (%p): nsICacheInfoChannel = %p",
+         mRequest.get(), mRequest->mCacheInfo.get()));
+  }
+
   // we have to mediate and use mRequest.
-  return mScriptLoader->OnStreamComplete(aLoader, mRequest, aStatus, mSRIStatus,
-                                         mSRIDataVerifier);
+  rv = mScriptLoader->OnStreamComplete(aLoader, mRequest, aStatus, mSRIStatus,
+                                       mSRIDataVerifier);
+
+  // In case of failure, clear the mCacheInfoChannel to avoid keeping it alive.
+  if (NS_FAILED(rv)) {
+    mRequest->mCacheInfo = nullptr;
+  }
+
+  return rv;
 }
 
 #undef LOG_ENABLED
diff --git a/dom/base/nsScriptLoader.h b/dom/base/nsScriptLoader.h
index 4c6a389cf413..672f143dbe4d 100644
--- a/dom/base/nsScriptLoader.h
+++ b/dom/base/nsScriptLoader.h
@@ -19,6 +19,7 @@
 #include "nsCycleCollectionParticipant.h"
 #include "nsTArray.h"
 #include "nsAutoPtr.h"
+#include "nsICacheInfoChannel.h"
 #include "nsIDocument.h"
 #include "nsIIncrementalStreamLoader.h"
 #include "nsURIHashKey.h"
@@ -99,7 +100,7 @@ public:
   }
 
   NS_DECL_CYCLE_COLLECTING_ISUPPORTS
-  NS_DECL_CYCLE_COLLECTION_CLASS(nsScriptLoadRequest)
+  NS_DECL_CYCLE_COLLECTION_SCRIPT_HOLDER_CLASS(nsScriptLoadRequest)
 
   bool IsModuleRequest() const
   {
@@ -187,6 +188,7 @@ public:
   }
 
   void MaybeCancelOffThreadScript();
+  void DropBytecodeCacheReferences();
 
   using super::getNext;
   using super::isInList;
@@ -208,6 +210,10 @@ public:
   void* mOffThreadToken;  // Off-thread parsing token.
   nsString mSourceMapURL; // Holds source map url for loaded scripts
 
+  // Holds the top-level JSScript that corresponds to the current source, once
+  // it is parsed, and planned to be saved in the bytecode cache.
+  JS::Heap<JSScript*> mScript;
+
   // Holds script text for non-inline scripts. Don't use nsString so we can give
   // ownership to jsapi.
   mozilla::Vector<char16_t> mScriptText;
@@ -225,6 +231,10 @@ public:
   const mozilla::CORSMode mCORSMode;
   const mozilla::dom::SRIMetadata mIntegrity;
   mozilla::net::ReferrerPolicy mReferrerPolicy;
+
+  // Holds the Cache information, which is used to register the bytecode
+  // on the cache entry, such that we can load it the next time.
+  nsCOMPtr<nsICacheInfoChannel> mCacheInfo;
 };
 
 class nsScriptLoadRequestList : private mozilla::LinkedList<nsScriptLoadRequest>
@@ -468,6 +478,11 @@ public:
                             nsresult aSRIStatus,
                             mozilla::dom::SRICheckDataVerifier* aSRIDataVerifier);
 
+  /**
+   * Returns wether any request is queued, and not executed yet.
+   */
+  bool HasPendingRequests();
+
   /**
    * Processes any pending requests that are ready for processing.
    */
@@ -537,6 +552,13 @@ public:
     return mDocument->GetDocGroup();
   }
 
+  /**
+   * Register the fact that we saw the load event, and that we need to save the
+   * bytecode at the next loop cycle unless new scripts are waiting in the
+   * pipeline.
+   */
+  void LoadEventFired();
+
 private:
   virtual ~nsScriptLoader();
 
@@ -622,6 +644,30 @@ private:
                            nsScriptLoadRequest* aRequest);
   nsresult EvaluateScript(nsScriptLoadRequest* aRequest);
 
+  /**
+   * Queue the current script load request to be saved, when the page
+   * initialization ends. The page initialization end is defined as being the
+   * time when the load event got received, and when no more scripts are waiting
+   * to be executed.
+   */
+  void RegisterForBytecodeEncoding(nsScriptLoadRequest* aRequest);
+
+  /**
+   * Check if all conditions are met, i-e that the onLoad event fired and that
+   * no more script have to be processed.  If all conditions are met, queue an
+   * event to encode all the bytecode and save them on the cache.
+   */
+  void MaybeTriggerBytecodeEncoding();
+
+  /**
+   * Iterate over all script load request and save the bytecode of executed
+   * functions on the cache provided by the channel.
+   */
+  void EncodeBytecode();
+  void EncodeRequestBytecode(JSContext* aCx, nsScriptLoadRequest* aRequest);
+
+  void GiveUpBytecodeEncoding();
+
   already_AddRefed<nsIScriptGlobalObject> GetScriptGlobalObject();
   nsresult FillCompileOptionsForRequest(const mozilla::dom::AutoJSAPI& jsapi,
                                         nsScriptLoadRequest* aRequest,
@@ -676,6 +722,10 @@ private:
   nsScriptLoadRequestList mXSLTRequests;
   RefPtr<nsScriptLoadRequest> mParserBlockingRequest;
 
+  // List of script load request that are holding a buffer which has to be saved
+  // on the cache.
+  nsScriptLoadRequestList mBytecodeEncodingQueue;
+
   // In mRequests, the additional information here is stored by the element.
   struct PreloadInfo {
     RefPtr<nsScriptLoadRequest> mRequest;
@@ -709,6 +759,7 @@ private:
   bool mDeferEnabled;
   bool mDocumentParsingDone;
   bool mBlockingDOMContentLoaded;
+  bool mLoadEventFired;
 
   // Module map
   nsRefPtrHashtable<nsURIHashKey, mozilla::GenericPromise::Private> mFetchingModules;
diff --git a/layout/base/nsDocumentViewer.cpp b/layout/base/nsDocumentViewer.cpp
index 102e0968790f..67aa1dd07aa8 100644
--- a/layout/base/nsDocumentViewer.cpp
+++ b/layout/base/nsDocumentViewer.cpp
@@ -1082,6 +1082,12 @@ nsDocumentViewer::LoadComplete(nsresult aStatus)
     }
   }
 
+  // Release the JS bytecode cache from its wait on the load event, and
+  // potentially dispatch the encoding of the bytecode.
+  if (mDocument && mDocument->ScriptLoader()) {
+    mDocument->ScriptLoader()->LoadEventFired();
+  }
+
   nsJSContext::LoadEnd();
 
   // It's probably a good idea to GC soon since we have finished loading.