diff --git a/ipc/glue/MessageChannel.cpp b/ipc/glue/MessageChannel.cpp
index 14d99a2db82b..5681468796ec 100644
--- a/ipc/glue/MessageChannel.cpp
+++ b/ipc/glue/MessageChannel.cpp
@@ -704,23 +704,6 @@ MessageChannel::Send(Message* aMsg, Message* aReply)
     IPC_ASSERT(mAwaitingSyncReplyPriority <= aMsg->priority(),
                "nested sync message sends must be of increasing priority");
 
-    AutoSetValue<bool> replies(mAwaitingSyncReply, true);
-    AutoSetValue<int> prio(mAwaitingSyncReplyPriority, aMsg->priority());
-    AutoEnterTransaction transact(this);
-    aMsg->set_transaction_id(mCurrentTransaction);
-
-    if (!SendAndWait(aMsg, aReply))
-        return false;
-
-    NS_ABORT_IF_FALSE(aReply->is_sync(), "reply is not sync");
-    return true;
-}
-
-bool
-MessageChannel::SendAndWait(Message* aMsg, Message* aReply)
-{
-    mMonitor->AssertCurrentThreadOwns();
-
     nsAutoPtr<Message> msg(aMsg);
 
     if (!Connected()) {
@@ -733,6 +716,13 @@ MessageChannel::SendAndWait(Message* aMsg, Message* aReply)
     DebugOnly<int32_t> replySeqno = msg->seqno();
     DebugOnly<msgid_t> replyType = msg->type() + 1;
 
+    AutoSetValue<bool> replies(mAwaitingSyncReply, true);
+    AutoSetValue<int> prio(mAwaitingSyncReplyPriority, msg->priority());
+    AutoEnterTransaction transact(this, msg->seqno());
+
+    int32_t transaction = mCurrentTransaction;
+    msg->set_transaction_id(transaction);
+
     mLink->SendMessage(msg.forget());
 
     while (true) {
@@ -770,6 +760,7 @@ MessageChannel::SendAndWait(Message* aMsg, Message* aReply)
 
             MOZ_ASSERT(mRecvd->type() == replyType, "wrong reply type");
             MOZ_ASSERT(mRecvd->seqno() == replySeqno);
+            MOZ_ASSERT(mRecvd->is_sync());
 
             *aReply = Move(*mRecvd);
             mRecvd = nullptr;
diff --git a/ipc/glue/MessageChannel.h b/ipc/glue/MessageChannel.h
index af770a7b5106..323e0450bc19 100644
--- a/ipc/glue/MessageChannel.h
+++ b/ipc/glue/MessageChannel.h
@@ -211,20 +211,6 @@ class MessageChannel : HasResultCodes
     // Send OnChannelConnected notification to listeners.
     void DispatchOnChannelConnected();
 
-    // Any protocol that requires blocking until a reply arrives, will send its
-    // outgoing message through this function. Currently, two protocols do this:
-    //
-    //  sync, which can only initiate messages from child to parent.
-    //  urgent, which can only initiate messages from parent to child.
-    //
-    // SendAndWait() expects that the worker thread owns the monitor, and that
-    // the message has been prepared to be sent over the link. It returns as
-    // soon as a reply has been received, or an error has occurred.
-    //
-    // Note that while the child is blocked waiting for a sync reply, it can wake
-    // up to process urgent calls from the parent.
-    bool SendAndWait(Message* aMsg, Message* aReply);
-
     bool InterruptEventOccurred();
 
     bool ProcessPendingRequest(const Message &aUrgent);
@@ -501,13 +487,13 @@ class MessageChannel : HasResultCodes
     class AutoEnterTransaction
     {
       public:
-       explicit AutoEnterTransaction(MessageChannel *aChan)
+       explicit AutoEnterTransaction(MessageChannel *aChan, int32_t aMsgSeqno)
         : mChan(aChan),
           mOldTransaction(mChan->mCurrentTransaction)
        {
            mChan->mMonitor->AssertCurrentThreadOwns();
            if (mChan->mCurrentTransaction == 0)
-               mChan->mCurrentTransaction = mChan->NextSeqno();
+               mChan->mCurrentTransaction = aMsgSeqno;
        }
        explicit AutoEnterTransaction(MessageChannel *aChan, const Message &aMessage)
         : mChan(aChan),