diff --git a/content/base/src/nsXMLHttpRequest.cpp b/content/base/src/nsXMLHttpRequest.cpp
index a7b268a9557..16827a349be 100644
--- a/content/base/src/nsXMLHttpRequest.cpp
+++ b/content/base/src/nsXMLHttpRequest.cpp
@@ -2991,6 +2991,14 @@ nsXMLHttpRequest::Send(nsIVariant* aVariant, const Nullable<RequestBody>& aBody)
         if (scheme.LowerCaseEqualsLiteral("app") ||
             scheme.LowerCaseEqualsLiteral("jar")) {
           mIsMappedArrayBuffer = true;
+          if (XRE_GetProcessType() != GeckoProcessType_Default) {
+            nsCOMPtr<nsIJARChannel> jarChannel = do_QueryInterface(mChannel);
+            // For memory mapping from child process, we need to get file
+            // descriptor of the JAR file opened remotely on the parent proess.
+            // Set this to make sure that file descriptor can be obtained by
+            // child process.
+            jarChannel->EnsureChildFd();
+          }
         }
       }
     }
diff --git a/modules/libjar/nsIJARChannel.idl b/modules/libjar/nsIJARChannel.idl
index e0fcc833d3e..8d68066761f 100644
--- a/modules/libjar/nsIJARChannel.idl
+++ b/modules/libjar/nsIJARChannel.idl
@@ -7,7 +7,7 @@
 
 interface nsIFile;
 
-[scriptable, builtinclass, uuid(063e9698-ec67-4fe2-aa19-d21505beaa61)]
+[scriptable, builtinclass, uuid(5a4f8df0-3bd9-45c2-9db9-67e74c3dd47d)]
 interface nsIJARChannel : nsIChannel
 {
     /**
@@ -27,4 +27,11 @@ interface nsIJARChannel : nsIChannel
      * Returns the JAR file.
      */
     readonly attribute nsIFile jarFile;
+
+    /**
+     * For child process, set this to make sure that a valid file descriptor of
+     * JAR file is always provided when calling NSPRFileDesc().
+     * Must be set before Open() or AsyncOpen() to be effective.
+     */
+    void ensureChildFd();
 };
diff --git a/modules/libjar/nsJARChannel.cpp b/modules/libjar/nsJARChannel.cpp
index 64373f81005..e7c283c8e04 100644
--- a/modules/libjar/nsJARChannel.cpp
+++ b/modules/libjar/nsJARChannel.cpp
@@ -199,6 +199,7 @@ nsJARChannel::nsJARChannel()
     , mIsPending(false)
     , mIsUnsafe(true)
     , mOpeningRemote(false)
+    , mEnsureChildFd(false)
 {
 #if defined(PR_LOGGING)
     if (!gJarProtocolLog)
@@ -356,7 +357,7 @@ nsJARChannel::LookupFile()
             mJarFile = remoteFile;
 
             nsIZipReaderCache *jarCache = gJarHandler->JarCache();
-            if (jarCache) {
+            if (jarCache && !mEnsureChildFd) {
                 bool cached = false;
                 rv = jarCache->IsCached(mJarFile, &cached);
                 if (NS_SUCCEEDED(rv) && cached) {
@@ -368,7 +369,8 @@ nsJARChannel::LookupFile()
 
             mOpeningRemote = true;
 
-            if (gJarHandler->RemoteOpenFileInProgress(remoteFile, this)) {
+            if (gJarHandler->RemoteOpenFileInProgress(remoteFile, this) &&
+                !mEnsureChildFd) {
                 // JarHandler will trigger OnRemoteFileOpen() after the first
                 // request for this file completes and we'll get a JAR cache
                 // hit.
@@ -865,6 +867,13 @@ nsJARChannel::GetJarFile(nsIFile **aFile)
     return NS_OK;
 }
 
+NS_IMETHODIMP
+nsJARChannel::EnsureChildFd()
+{
+    mEnsureChildFd = true;
+    return NS_OK;
+}
+
 //-----------------------------------------------------------------------------
 // nsIDownloadObserver
 //-----------------------------------------------------------------------------
@@ -1035,6 +1044,11 @@ nsJARChannel::OnStopRequest(nsIRequest *req, nsISupports *ctx, nsresult status)
     mCallbacks = 0;
     mProgressSink = 0;
 
+    if (mOpeningRemote) {
+      // To deallocate file descriptor by RemoteOpenFileChild destructor.
+      mJarFile = nullptr;
+    }
+
     return NS_OK;
 }
 
diff --git a/modules/libjar/nsJARChannel.h b/modules/libjar/nsJARChannel.h
index bfaf294de55..796494353f0 100644
--- a/modules/libjar/nsJARChannel.h
+++ b/modules/libjar/nsJARChannel.h
@@ -93,6 +93,7 @@ private:
     bool                            mIsPending;
     bool                            mIsUnsafe;
     bool                            mOpeningRemote;
+    bool                            mEnsureChildFd;
 
     nsCOMPtr<nsIStreamListener>     mDownloader;
     nsCOMPtr<nsIInputStreamPump>    mPump;
diff --git a/netwerk/ipc/RemoteOpenFileChild.cpp b/netwerk/ipc/RemoteOpenFileChild.cpp
index 707ad23613c..ec445a12d67 100644
--- a/netwerk/ipc/RemoteOpenFileChild.cpp
+++ b/netwerk/ipc/RemoteOpenFileChild.cpp
@@ -72,10 +72,20 @@ NS_IMPL_ISUPPORTS(RemoteOpenFileChild,
 
 RemoteOpenFileChild::RemoteOpenFileChild(const RemoteOpenFileChild& other)
   : mTabChild(other.mTabChild)
-  , mNSPRFileDesc(other.mNSPRFileDesc)
+  , mNSPRFileDesc(nullptr)
   , mAsyncOpenCalled(other.mAsyncOpenCalled)
-  , mNSPROpenCalled(other.mNSPROpenCalled)
 {
+#if defined(XP_WIN) || defined(MOZ_WIDGET_COCOA)
+  // Windows/OSX desktop builds skip remoting, so the file descriptor should
+  // be nullptr here.
+  MOZ_ASSERT(!other.mNSPRFileDesc);
+#else
+  if (other.mNSPRFileDesc) {
+    PROsfd osfd = dup(PR_FileDesc2NativeHandle(other.mNSPRFileDesc));
+    mNSPRFileDesc = PR_ImportFile(osfd);
+  }
+#endif
+
   // Note: don't clone mListener or we'll have a refcount leak.
   other.mURI->Clone(getter_AddRefs(mURI));
   if (other.mAppURI) {
@@ -123,8 +133,6 @@ RemoteOpenFileChild::~RemoteOpenFileChild()
   }
 
   if (mNSPRFileDesc) {
-    // If we handed out fd we shouldn't have pointer to it any more.
-    MOZ_ASSERT(!mNSPROpenCalled);
     // PR_Close both closes the file and deallocates the PRFileDesc
     PR_Close(mNSPRFileDesc);
   }
@@ -347,11 +355,6 @@ RemoteOpenFileChild::Clone(nsIFile **file)
   *file = new RemoteOpenFileChild(*this);
   NS_ADDREF(*file);
 
-  // if we transferred ownership of file to clone, forget our pointer.
-  if (mNSPRFileDesc) {
-    mNSPRFileDesc = nullptr;
-  }
-
   return NS_OK;
 }
 
@@ -370,21 +373,13 @@ RemoteOpenFileChild::OpenNSPRFileDesc(int32_t aFlags, int32_t aMode,
     return NS_ERROR_NOT_AVAILABLE;
   }
 
-  // Unlike regular nsIFile we can't (easily) support multiple open()s.
-  if (mNSPROpenCalled) {
-    return NS_ERROR_ALREADY_OPENED;
-  }
-
   if (!mNSPRFileDesc) {
-    // client skipped AsyncRemoteFileOpen() or didn't wait for result, or this
-    // object has been cloned
+    // Client skipped AsyncRemoteFileOpen() or didn't wait for result.
     return NS_ERROR_NOT_AVAILABLE;
   }
 
-  // hand off ownership (i.e responsibility to PR_Close() file handle) to caller
-  *aRetval = mNSPRFileDesc;
-  mNSPRFileDesc = nullptr;
-  mNSPROpenCalled = true;
+  PROsfd osfd = dup(PR_FileDesc2NativeHandle(mNSPRFileDesc));
+  *aRetval = PR_ImportFile(osfd);
 
   return NS_OK;
 #endif
diff --git a/netwerk/protocol/app/AppProtocolHandler.cpp b/netwerk/protocol/app/AppProtocolHandler.cpp
index 94a5f32a1e8..7a1443b0bcb 100644
--- a/netwerk/protocol/app/AppProtocolHandler.cpp
+++ b/netwerk/protocol/app/AppProtocolHandler.cpp
@@ -127,6 +127,11 @@ NS_IMETHODIMP DummyChannel::GetJarFile(nsIFile* *aFile)
   return NS_ERROR_NOT_IMPLEMENTED;
 }
 
+NS_IMETHODIMP DummyChannel::EnsureChildFd()
+{
+  return NS_ERROR_NOT_IMPLEMENTED;
+}
+
 NS_IMETHODIMP DummyChannel::Run()
 {
   nsresult rv = mListener->OnStartRequest(this, mListenerContext);