diff --git a/netwerk/protocol/http/public/nsIHttpProtocolHandler.idl b/netwerk/protocol/http/public/nsIHttpProtocolHandler.idl
index 13f81d21834..4900e5886fe 100644
--- a/netwerk/protocol/http/public/nsIHttpProtocolHandler.idl
+++ b/netwerk/protocol/http/public/nsIHttpProtocolHandler.idl
@@ -158,5 +158,12 @@ interface nsIHttpProtocolHandler : nsIProxiedProtocolHandler
  */
 #define NS_HTTP_ON_EXAMINE_MERGED_RESPONSE_TOPIC "http-on-examine-merged-response"
 
+/**
+ * The observer of this topic is notified before data is read from the cache.
+ * The notification is sent if and only if there is no network communication
+ * at all.
+ */
+#define NS_HTTP_ON_EXAMINE_CACHED_RESPONSE_TOPIC "http-on-examine-cached-response"
+
 
 %}
diff --git a/netwerk/protocol/http/src/nsHttpChannel.cpp b/netwerk/protocol/http/src/nsHttpChannel.cpp
index 970abf23771..21ddb816079 100644
--- a/netwerk/protocol/http/src/nsHttpChannel.cpp
+++ b/netwerk/protocol/http/src/nsHttpChannel.cpp
@@ -255,11 +255,19 @@ nsHttpChannel::Init(nsIURI *uri,
 //-----------------------------------------------------------------------------
 
 nsresult
-nsHttpChannel::AsyncCall(nsAsyncCallback funcPtr)
+nsHttpChannel::AsyncCall(nsAsyncCallback funcPtr,
+                         nsRunnableMethod<nsHttpChannel> **retval)
 {
-    nsCOMPtr<nsIRunnable> event =
+    nsresult rv;
+
+    nsRefPtr<nsRunnableMethod<nsHttpChannel> > event =
             new nsRunnableMethod<nsHttpChannel>(this, funcPtr);
-    return NS_DispatchToCurrentThread(event);
+    rv = NS_DispatchToCurrentThread(event);
+    if (NS_SUCCEEDED(rv) && retval) {
+        *retval = event;
+    }
+
+    return rv;
 }
 
 PRBool
@@ -341,7 +349,15 @@ nsHttpChannel::Connect(PRBool firstTime)
 
         // read straight from the cache if possible...
         if (mCachedContentIsValid) {
-            return ReadFromCache();
+            nsRunnableMethod<nsHttpChannel> *event = nsnull;
+            if (!mCachedContentIsPartial) {
+                AsyncCall(&nsHttpChannel::AsyncOnExamineCachedResponse, &event);
+            }
+            rv = ReadFromCache();
+            if (NS_FAILED(rv) && event) {
+                event->Revoke();
+            }
+            return rv;
         }
         else if (mLoadFlags & LOAD_ONLY_FROM_CACHE) {
             // the cache contains the requested resource, but it must be 
@@ -5591,3 +5607,8 @@ nsHttpChannel::DetermineStoragePolicy()
     return policy;
 }
 
+void
+nsHttpChannel::AsyncOnExamineCachedResponse()
+{
+    gHttpHandler->OnExamineCachedResponse(this);
+}
diff --git a/netwerk/protocol/http/src/nsHttpChannel.h b/netwerk/protocol/http/src/nsHttpChannel.h
index c26693498ac..a3523af8c29 100644
--- a/netwerk/protocol/http/src/nsHttpChannel.h
+++ b/netwerk/protocol/http/src/nsHttpChannel.h
@@ -153,7 +153,10 @@ private:
     }
 
     // AsyncCall may be used to call a member function asynchronously.
-    nsresult AsyncCall(nsAsyncCallback funcPtr);
+    // retval isn't refcounted and is set only when event was successfully
+    // posted, the event is returned for the purpose of cancelling when needed
+    nsresult AsyncCall(nsAsyncCallback funcPtr,
+                       nsRunnableMethod<nsHttpChannel> **retval = nsnull);
 
     PRBool   RequestIsConditional();
     nsresult Connect(PRBool firstTime = PR_TRUE);
@@ -205,6 +208,7 @@ private:
     nsresult InstallOfflineCacheListener();
     void     MaybeInvalidateCacheEntryForSubsequentGet();
     nsCacheStoragePolicy DetermineStoragePolicy();
+    void     AsyncOnExamineCachedResponse();
 
     // Handle the bogus Content-Encoding Apache sometimes sends
     void ClearBogusContentEncodingIfNeeded();
diff --git a/netwerk/protocol/http/src/nsHttpHandler.h b/netwerk/protocol/http/src/nsHttpHandler.h
index 6ccd3de57c5..c12973a4bb8 100644
--- a/netwerk/protocol/http/src/nsHttpHandler.h
+++ b/netwerk/protocol/http/src/nsHttpHandler.h
@@ -195,6 +195,13 @@ public:
     // channel's and the global redirect observers.
     nsresult OnChannelRedirect(nsIChannel* oldChan, nsIChannel* newChan,
                                PRUint32 flags);
+
+    // Called by the channel when the response is read from the cache without
+    // communicating with the server.
+    void OnExamineCachedResponse(nsIHttpChannel *chan)
+    {
+        NotifyObservers(chan, NS_HTTP_ON_EXAMINE_CACHED_RESPONSE_TOPIC);
+    }
 private:
 
     //