--- a/netwerk/protocol/http/nsHttpChannel.cpp
+++ b/netwerk/protocol/http/nsHttpChannel.cpp
@@ -4289,16 +4289,21 @@ nsHttpChannel::AsyncOpen(nsIStreamListen
     // Remember the cookie header that was set, if any
     const char *cookieHeader = mRequestHead.PeekHeader(nsHttp::Cookie);
     if (cookieHeader) {
         mUserSetCookieHeader = cookieHeader;
     }
 
     AddCookiesToRequest();
 
+    // notify "http-on-opening-request" observers, but not if this is a redirect
+    if (!(mLoadFlags & LOAD_REPLACE)) {
+        gHttpHandler->OnOpeningRequest(this);
+    }
+
     mIsPending = true;
     mWasOpened = true;
 
     mListener = listener;
     mListenerContext = context;
 
     // add ourselves to the load group.  from this point forward, we'll report
     // all failures asynchronously.

--- a/netwerk/protocol/http/nsHttpHandler.h
+++ b/netwerk/protocol/http/nsHttpHandler.h
@@ -172,16 +172,22 @@ public:
     nsresult GetStreamConverterService(nsIStreamConverterService **);
     nsresult GetIOService(nsIIOService** service);
     nsICookieService * GetCookieService(); // not addrefed
     nsIStrictTransportSecurityService * GetSTSService();
 
     // callable from socket thread only
     uint32_t Get32BitsOfPseudoRandom();
 
+    // Called by the channel synchronously during asyncOpen
+    void OnOpeningRequest(nsIHttpChannel *chan)
+    {
+        NotifyObservers(chan, NS_HTTP_ON_OPENING_REQUEST_TOPIC);
+    }
+
     // Called by the channel before writing a request
     void OnModifyRequest(nsIHttpChannel *chan)
     {
         NotifyObservers(chan, NS_HTTP_ON_MODIFY_REQUEST_TOPIC);
     }
 
     // Called by the channel once headers are available
     void OnExamineResponse(nsIHttpChannel *chan)

--- a/netwerk/protocol/http/nsIHttpProtocolHandler.idl
+++ b/netwerk/protocol/http/nsIHttpProtocolHandler.idl
@@ -62,16 +62,34 @@ interface nsIHttpProtocolHandler : nsIPr
  * nsIObserver notification corresponding to startup category.  Services
  * registered under the startup category will receive this observer topic at
  * startup if they implement nsIObserver.  The "subject" of the notification
  * is the nsIHttpProtocolHandler instance.
  */
 #define NS_HTTP_STARTUP_TOPIC "http-startup"
 
 /**
+ * This observer topic is notified when an HTTP channel is opened.
+ * It is similar to http-on-modify-request, except that
+ * 1) The notification is guaranteed to occur before on-modify-request, during
+ *    the AsyncOpen call itself.
+ * 2) It only occurs for the initial open of a channel, not for internal
+ *    asyncOpens that happen during redirects, etc.
+ * 3) Some information (most notably nsIProxiedChannel.proxyInfo) may not be set
+ *    on the channel object yet.
+ *
+ * The "subject" of the notification is the nsIHttpChannel instance.
+ *
+ * Generally the 'http-on-modify-request' notification is preferred unless the
+ * synchronous, during-asyncOpen behavior that this notification provides is
+ * required.
+ */
+#define NS_HTTP_ON_OPENING_REQUEST_TOPIC "http-on-opening-request"
+
+/**
  * Before an HTTP request is sent to the server, this observer topic is
  * notified.  The observer of this topic can then choose to set any additional
  * headers for this request before the request is actually sent to the server.
  * The "subject" of the notification is the nsIHttpChannel instance.
  */
 #define NS_HTTP_ON_MODIFY_REQUEST_TOPIC "http-on-modify-request"
 
 /**