diff --git a/ipc/glue/IPCMessageUtils.h b/ipc/glue/IPCMessageUtils.h
index 04be310761..e30a5fa952 100644
--- a/ipc/glue/IPCMessageUtils.h
+++ b/ipc/glue/IPCMessageUtils.h
@@ -385,6 +385,18 @@ struct ParamTraits<nsLiteralCString> : ParamTraits<nsACString>
   typedef nsLiteralCString paramType;
 };
 
+template <>
+struct ParamTraits<nsDependentSubstring> : ParamTraits<nsAString>
+{
+  typedef nsDependentSubstring paramType;
+};
+
+template <>
+struct ParamTraits<nsDependentCSubstring> : ParamTraits<nsACString>
+{
+  typedef nsDependentCSubstring paramType;
+};
+
 #ifdef MOZILLA_INTERNAL_API
 
 template<>
diff --git a/ipc/ipdl/ipdl/builtin.py b/ipc/ipdl/ipdl/builtin.py
index 3a49b5c7ee..3c332cf838 100644
--- a/ipc/ipdl/ipdl/builtin.py
+++ b/ipc/ipdl/ipdl/builtin.py
@@ -36,6 +36,8 @@ Types = (
     'nsresult',
     'nsString',
     'nsCString',
+    'nsDependentSubstring',
+    'nsDependentCSubstring',
     'mozilla::ipc::Shmem',
     'mozilla::ipc::FileDescriptor'
 )
diff --git a/netwerk/protocol/wyciwyg/PWyciwygChannel.ipdl b/netwerk/protocol/wyciwyg/PWyciwygChannel.ipdl
index b1eabc4fdb..939f106988 100644
--- a/netwerk/protocol/wyciwyg/PWyciwygChannel.ipdl
+++ b/netwerk/protocol/wyciwyg/PWyciwygChannel.ipdl
@@ -35,7 +35,7 @@ parent:
   async AppData(SerializedLoadContext loadContext, PBrowserOrId browser);
 
   // methods corresponding to those of nsIWyciwygChannel
-  async WriteToCacheEntry(nsString data);
+  async WriteToCacheEntry(nsDependentSubstring data);
   async CloseCacheEntry(nsresult reason);
   async SetCharsetAndSource(int32_t source, nsCString charset);
   async SetSecurityInfo(nsCString securityInfo);
diff --git a/netwerk/protocol/wyciwyg/WyciwygChannelChild.cpp b/netwerk/protocol/wyciwyg/WyciwygChannelChild.cpp
index 2a794df3b4..9966df90fc 100644
--- a/netwerk/protocol/wyciwyg/WyciwygChannelChild.cpp
+++ b/netwerk/protocol/wyciwyg/WyciwygChannelChild.cpp
@@ -689,8 +689,22 @@ WyciwygChannelChild::WriteToCacheEntry(const nsAString & aData)
     mSentAppData = true;
   }
 
-  SendWriteToCacheEntry(PromiseFlatString(aData));
   mState = WCC_ONWRITE;
+
+  // Give ourselves a megabyte of headroom for the message size. Convert bytes
+  // to wide chars.
+  static const size_t kMaxMessageSize = (IPC::Channel::kMaximumMessageSize - 1024) / 2;
+
+  size_t curIndex = 0;
+  size_t charsRemaining = aData.Length();
+  do {
+    size_t chunkSize = std::min(charsRemaining, kMaxMessageSize);
+    SendWriteToCacheEntry(Substring(aData, curIndex, chunkSize));
+
+    charsRemaining -= chunkSize;
+    curIndex += chunkSize;
+  } while (charsRemaining != 0);
+
   return NS_OK;
 }
 
diff --git a/netwerk/protocol/wyciwyg/WyciwygChannelParent.cpp b/netwerk/protocol/wyciwyg/WyciwygChannelParent.cpp
index be00c2d869..6a7d34e1dc 100644
--- a/netwerk/protocol/wyciwyg/WyciwygChannelParent.cpp
+++ b/netwerk/protocol/wyciwyg/WyciwygChannelParent.cpp
@@ -213,7 +213,7 @@ WyciwygChannelParent::RecvAsyncOpen(const URIParams& aOriginal,
 }
 
 bool
-WyciwygChannelParent::RecvWriteToCacheEntry(const nsString& data)
+WyciwygChannelParent::RecvWriteToCacheEntry(const nsDependentSubstring& data)
 {
   if (!mReceivedAppData) {
     printf_stderr("WyciwygChannelParent::RecvWriteToCacheEntry: FATAL ERROR: didn't receive app data\n");
diff --git a/netwerk/protocol/wyciwyg/WyciwygChannelParent.h b/netwerk/protocol/wyciwyg/WyciwygChannelParent.h
index be009487b1..7a0a948271 100644
--- a/netwerk/protocol/wyciwyg/WyciwygChannelParent.h
+++ b/netwerk/protocol/wyciwyg/WyciwygChannelParent.h
@@ -45,7 +45,7 @@ protected:
                              const uint32_t& loadFlags,
                              const IPC::SerializedLoadContext& loadContext,
                              const PBrowserOrId &parent) override;
-  virtual bool RecvWriteToCacheEntry(const nsString& data) override;
+  virtual bool RecvWriteToCacheEntry(const nsDependentSubstring& data) override;
   virtual bool RecvCloseCacheEntry(const nsresult& reason) override;
   virtual bool RecvSetCharsetAndSource(const int32_t& source,
                                        const nsCString& charset) override;