--- a/netwerk/protocol/http/HttpBaseChannel.cpp
+++ b/netwerk/protocol/http/HttpBaseChannel.cpp
@@ -198,16 +198,17 @@ HttpBaseChannel::HttpBaseChannel()
   , mAfterOnStartRequestBegun(false)
   , mTransferSize(0)
   , mDecodedBodySize(0)
   , mEncodedBodySize(0)
   , mContentWindowId(0)
   , mTopLevelOuterContentWindowId(0)
   , mRequireCORSPreflight(false)
   , mReportCollector(new ConsoleReportCollector())
+  , mAltDataLength(0)
   , mForceMainDocumentChannel(false)
   , mIsTrackingResource(false)
 {
   LOG(("Creating HttpBaseChannel @%p\n", this));
 
   // Subfields of unions cannot be targeted in an initializer list.
 #ifdef MOZ_VALGRIND
   // Zero the entire unions so that Valgrind doesn't complain when we send them
@@ -696,16 +697,21 @@ HttpBaseChannel::GetContentDispositionHe
 NS_IMETHODIMP
 HttpBaseChannel::GetContentLength(int64_t *aContentLength)
 {
   NS_ENSURE_ARG_POINTER(aContentLength);
 
   if (!mResponseHead)
     return NS_ERROR_NOT_AVAILABLE;
 
+  if (!mAvailableCachedAltDataType.IsEmpty()) {
+    *aContentLength = mAltDataLength;
+    return NS_OK;
+  }
+
   *aContentLength = mResponseHead->ContentLength();
   return NS_OK;
 }
 
 NS_IMETHODIMP
 HttpBaseChannel::SetContentLength(int64_t value)
 {
   NS_NOTYETIMPLEMENTED("HttpBaseChannel::SetContentLength");

--- a/netwerk/protocol/http/HttpBaseChannel.h
+++ b/netwerk/protocol/http/HttpBaseChannel.h
@@ -331,16 +331,17 @@ public:
     nsHttpRequestHead * GetRequestHead() { return &mRequestHead; }
 
     const NetAddr& GetSelfAddr() { return mSelfAddr; }
     const NetAddr& GetPeerAddr() { return mPeerAddr; }
 
     MOZ_MUST_USE nsresult OverrideSecurityInfo(nsISupports* aSecurityInfo);
 
 public: /* Necko internal use only... */
+    int64_t GetAltDataLength() { return mAltDataLength; }
     bool IsNavigation();
 
     // Return whether upon a redirect code of httpStatus for method, the
     // request method should be rewritten to GET.
     static bool ShouldRewriteRedirectToGET(uint32_t httpStatus,
                                            nsHttpRequestHead::ParsedMethodType method);
 
     // Like nsIEncodedChannel::DoApplyConversions except context is set to
@@ -599,16 +600,17 @@ protected:
   nsTArray<nsCString>               mUnsafeHeaders;
 
   nsCOMPtr<nsIConsoleReportCollector> mReportCollector;
 
   // Holds the name of the preferred alt-data type.
   nsCString mPreferredCachedAltDataType;
   // Holds the name of the alternative data type the channel returned.
   nsCString mAvailableCachedAltDataType;
+  int64_t   mAltDataLength;
 
   bool mForceMainDocumentChannel;
   bool mIsTrackingResource;
 
   nsID mChannelId;
 
   nsString mIntegrityMetadata;
 

--- a/netwerk/protocol/http/HttpChannelChild.cpp
+++ b/netwerk/protocol/http/HttpChannelChild.cpp
@@ -320,74 +320,78 @@ class StartRequestEvent : public Channel
                     const bool& aIsFromCache,
                     const bool& aCacheEntryAvailable,
                     const uint32_t& aCacheExpirationTime,
                     const nsCString& aCachedCharset,
                     const nsCString& aSecurityInfoSerialization,
                     const NetAddr& aSelfAddr,
                     const NetAddr& aPeerAddr,
                     const uint32_t& aCacheKey,
-                    const nsCString& altDataType)
+                    const nsCString& altDataType,
+                    const int64_t& altDataLen)
   : mChild(aChild)
   , mChannelStatus(aChannelStatus)
   , mResponseHead(aResponseHead)
   , mRequestHeaders(aRequestHeaders)
   , mUseResponseHead(aUseResponseHead)
   , mIsFromCache(aIsFromCache)
   , mCacheEntryAvailable(aCacheEntryAvailable)
   , mCacheExpirationTime(aCacheExpirationTime)
   , mCachedCharset(aCachedCharset)
   , mSecurityInfoSerialization(aSecurityInfoSerialization)
   , mSelfAddr(aSelfAddr)
   , mPeerAddr(aPeerAddr)
   , mCacheKey(aCacheKey)
   , mAltDataType(altDataType)
+  , mAltDataLen(altDataLen)
   {}
 
   void Run()
   {
     LOG(("StartRequestEvent [this=%p]\n", mChild));
     mChild->OnStartRequest(mChannelStatus, mResponseHead, mUseResponseHead,
                            mRequestHeaders, mIsFromCache, mCacheEntryAvailable,
                            mCacheExpirationTime, mCachedCharset,
                            mSecurityInfoSerialization, mSelfAddr, mPeerAddr,
-                           mCacheKey, mAltDataType);
+                           mCacheKey, mAltDataType, mAltDataLen);
   }
  private:
   HttpChannelChild* mChild;
   nsresult mChannelStatus;
   nsHttpResponseHead mResponseHead;
   nsHttpHeaderArray mRequestHeaders;
   bool mUseResponseHead;
   bool mIsFromCache;
   bool mCacheEntryAvailable;
   uint32_t mCacheExpirationTime;
   nsCString mCachedCharset;
   nsCString mSecurityInfoSerialization;
   NetAddr mSelfAddr;
   NetAddr mPeerAddr;
   uint32_t mCacheKey;
   nsCString mAltDataType;
+  int64_t mAltDataLen;
 };
 
 mozilla::ipc::IPCResult
 HttpChannelChild::RecvOnStartRequest(const nsresult& channelStatus,
                                      const nsHttpResponseHead& responseHead,
                                      const bool& useResponseHead,
                                      const nsHttpHeaderArray& requestHeaders,
                                      const bool& isFromCache,
                                      const bool& cacheEntryAvailable,
                                      const uint32_t& cacheExpirationTime,
                                      const nsCString& cachedCharset,
                                      const nsCString& securityInfoSerialization,
                                      const NetAddr& selfAddr,
                                      const NetAddr& peerAddr,
                                      const int16_t& redirectCount,
                                      const uint32_t& cacheKey,
-                                     const nsCString& altDataType)
+                                     const nsCString& altDataType,
+                                     const int64_t& altDataLen)
 {
   LOG(("HttpChannelChild::RecvOnStartRequest [this=%p]\n", this));
   // mFlushedForDiversion and mDivertingToParent should NEVER be set at this
   // stage, as they are set in the listener's OnStartRequest.
   MOZ_RELEASE_ASSERT(!mFlushedForDiversion,
     "mFlushedForDiversion should be unset before OnStartRequest!");
   MOZ_RELEASE_ASSERT(!mDivertingToParent,
     "mDivertingToParent should be unset before OnStartRequest!");
@@ -397,34 +401,35 @@ HttpChannelChild::RecvOnStartRequest(con
 
   mEventQ->RunOrEnqueue(new StartRequestEvent(this, channelStatus, responseHead,
                                               useResponseHead, requestHeaders,
                                               isFromCache, cacheEntryAvailable,
                                               cacheExpirationTime,
                                               cachedCharset,
                                               securityInfoSerialization,
                                               selfAddr, peerAddr, cacheKey,
-                                              altDataType));
+                                              altDataType, altDataLen));
   return IPC_OK();
 }
 
 void
 HttpChannelChild::OnStartRequest(const nsresult& channelStatus,
                                  const nsHttpResponseHead& responseHead,
                                  const bool& useResponseHead,
                                  const nsHttpHeaderArray& requestHeaders,
                                  const bool& isFromCache,
                                  const bool& cacheEntryAvailable,
                                  const uint32_t& cacheExpirationTime,
                                  const nsCString& cachedCharset,
                                  const nsCString& securityInfoSerialization,
                                  const NetAddr& selfAddr,
                                  const NetAddr& peerAddr,
                                  const uint32_t& cacheKey,
-                                 const nsCString& altDataType)
+                                 const nsCString& altDataType,
+                                 const int64_t& altDataLen)
 {
   LOG(("HttpChannelChild::OnStartRequest [this=%p]\n", this));
 
   // mFlushedForDiversion and mDivertingToParent should NEVER be set at this
   // stage, as they are set in the listener's OnStartRequest.
   MOZ_RELEASE_ASSERT(!mFlushedForDiversion,
     "mFlushedForDiversion should be unset before OnStartRequest!");
   MOZ_RELEASE_ASSERT(!mDivertingToParent,
@@ -445,16 +450,17 @@ HttpChannelChild::OnStartRequest(const n
   mIsFromCache = isFromCache;
   mCacheEntryAvailable = cacheEntryAvailable;
   mCacheExpirationTime = cacheExpirationTime;
   mCachedCharset = cachedCharset;
   mSelfAddr = selfAddr;
   mPeerAddr = peerAddr;
 
   mAvailableCachedAltDataType = altDataType;
+  mAltDataLength = altDataLen;
 
   mAfterOnStartRequestBegun = true;
 
   AutoEventEnqueuer ensureSerialDispatch(mEventQ);
 
   nsresult rv;
   nsCOMPtr<nsISupportsPRUint32> container =
     do_CreateInstance(NS_SUPPORTS_PRUINT32_CONTRACTID, &rv);

--- a/netwerk/protocol/http/HttpChannelChild.h
+++ b/netwerk/protocol/http/HttpChannelChild.h
@@ -119,17 +119,18 @@ protected:
                                              const bool& cacheEntryAvailable,
                                              const uint32_t& cacheExpirationTime,
                                              const nsCString& cachedCharset,
                                              const nsCString& securityInfoSerialization,
                                              const NetAddr& selfAddr,
                                              const NetAddr& peerAddr,
                                              const int16_t& redirectCount,
                                              const uint32_t& cacheKey,
-                                             const nsCString& altDataType) override;
+                                             const nsCString& altDataType,
+                                             const int64_t& altDataLen) override;
   mozilla::ipc::IPCResult RecvOnTransportAndData(const nsresult& channelStatus,
                                                  const nsresult& status,
                                                  const uint64_t& offset,
                                                  const uint32_t& count,
                                                  const nsCString& data) override;
   mozilla::ipc::IPCResult RecvOnStopRequest(const nsresult& statusCode, const ResourceTimingStruct& timing) override;
   mozilla::ipc::IPCResult RecvOnProgress(const int64_t& progress, const int64_t& progressMax) override;
   mozilla::ipc::IPCResult RecvOnStatus(const nsresult& status) override;
@@ -298,17 +299,18 @@ private:
                       const bool& isFromCache,
                       const bool& cacheEntryAvailable,
                       const uint32_t& cacheExpirationTime,
                       const nsCString& cachedCharset,
                       const nsCString& securityInfoSerialization,
                       const NetAddr& selfAddr,
                       const NetAddr& peerAddr,
                       const uint32_t& cacheKey,
-                      const nsCString& altDataType);
+                      const nsCString& altDataType,
+                      const int64_t& altDataLen);
   void MaybeDivertOnData(const nsCString& data,
                          const uint64_t& offset,
                          const uint32_t& count);
   void OnTransportAndData(const nsresult& channelStatus,
                           const nsresult& status,
                           const uint64_t& offset,
                           const uint32_t& count,
                           const nsCString& data);

--- a/netwerk/protocol/http/HttpChannelParent.cpp
+++ b/netwerk/protocol/http/HttpChannelParent.cpp
@@ -1183,32 +1183,34 @@ HttpChannelParent::OnStartRequest(nsIReq
     nsresult rv = container->GetData(&cacheKeyValue);
     if (NS_FAILED(rv)) {
       return rv;
     }
   }
 
   nsAutoCString altDataType;
   chan->GetAlternativeDataType(altDataType);
+  int64_t altDataLen = chan->GetAltDataLength();
 
   // !!! We need to lock headers and please don't forget to unlock them !!!
   requestHead->Enter();
   nsresult rv = NS_OK;
   if (mIPCClosed ||
       !SendOnStartRequest(channelStatus,
                           responseHead ? *responseHead : nsHttpResponseHead(),
                           !!responseHead,
                           requestHead->Headers(),
                           isFromCache,
                           mCacheEntry ? true : false,
                           expirationTime, cachedCharset, secInfoSerialization,
                           chan->GetSelfAddr(), chan->GetPeerAddr(),
                           redirectCount,
                           cacheKeyValue,
-                          altDataType))
+                          altDataType,
+                          altDataLen))
   {
     rv = NS_ERROR_UNEXPECTED;
   }
   requestHead->Exit();
   return rv;
 }
 
 NS_IMETHODIMP

--- a/netwerk/protocol/http/PHttpChannel.ipdl
+++ b/netwerk/protocol/http/PHttpChannel.ipdl
@@ -103,17 +103,18 @@ child:
                        bool                cacheEntryAvailable,
                        uint32_t            cacheExpirationTime,
                        nsCString           cachedCharset,
                        nsCString           securityInfoSerialization,
                        NetAddr             selfAddr,
                        NetAddr             peerAddr,
                        int16_t             redirectCount,
                        uint32_t            cacheKey,
-                       nsCString           altDataType);
+                       nsCString           altDataType,
+                       int64_t             altDataLength);
 
   // Combines a single OnDataAvailable and its associated OnProgress &
   // OnStatus calls into one IPDL message
   async OnTransportAndData(nsresult  channelStatus,
                            nsresult  transportStatus,
                            uint64_t  offset,
                            uint32_t  count,
                            nsCString data);

--- a/netwerk/protocol/http/nsHttpChannel.cpp
+++ b/netwerk/protocol/http/nsHttpChannel.cpp
@@ -4620,22 +4620,20 @@ nsHttpChannel::OpenCacheInputStream(nsIC
     // If an alternate representation was requested, try to open the alt
     // input stream.
     if (!mPreferredCachedAltDataType.IsEmpty()) {
         rv = cacheEntry->OpenAlternativeInputStream(mPreferredCachedAltDataType,
                                                     getter_AddRefs(stream));
         if (NS_SUCCEEDED(rv)) {
             // We have succeeded.
             mAvailableCachedAltDataType = mPreferredCachedAltDataType;
-            // Clear the header.
-            mCachedResponseHead->SetContentLength(-1);
             // Set the correct data size on the channel.
             int64_t altDataSize;
             if (NS_SUCCEEDED(cacheEntry->GetAltDataSize(&altDataSize))) {
-                mCachedResponseHead->SetContentLength(altDataSize);
+                mAltDataLength = altDataSize;
             }
         }
     }
 
     if (!stream) {
         rv = cacheEntry->OpenInputStream(0, getter_AddRefs(stream));
     }