diff --git a/netwerk/protocol/http/HttpBaseChannel.cpp b/netwerk/protocol/http/HttpBaseChannel.cpp
index 7cb49d120e98..05c3edb926fb 100644
--- a/netwerk/protocol/http/HttpBaseChannel.cpp
+++ b/netwerk/protocol/http/HttpBaseChannel.cpp
@@ -73,6 +73,7 @@ HttpBaseChannel::HttpBaseChannel()
   , mInheritApplicationCache(PR_TRUE)
   , mChooseApplicationCache(PR_FALSE)
   , mLoadedFromApplicationCache(PR_FALSE)
+  , mChannelIsForDownload(PR_FALSE)
 {
   LOG(("Creating HttpBaseChannel @%x\n", this));
 
@@ -935,6 +936,20 @@ HttpBaseChannel::GetCanceled(PRBool *aCanceled)
   return NS_OK;
 }
 
+NS_IMETHODIMP
+HttpBaseChannel::GetChannelIsForDownload(PRBool *aChannelIsForDownload)
+{
+  *aChannelIsForDownload = mChannelIsForDownload;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+HttpBaseChannel::SetChannelIsForDownload(PRBool aChannelIsForDownload)
+{
+  mChannelIsForDownload = aChannelIsForDownload;
+  return NS_OK;
+}
+
 //-----------------------------------------------------------------------------
 // HttpBaseChannel::nsISupportsPriority
 //-----------------------------------------------------------------------------
diff --git a/netwerk/protocol/http/HttpBaseChannel.h b/netwerk/protocol/http/HttpBaseChannel.h
index 180018da9429..ebb523f2a787 100644
--- a/netwerk/protocol/http/HttpBaseChannel.h
+++ b/netwerk/protocol/http/HttpBaseChannel.h
@@ -165,6 +165,8 @@ public:
   NS_IMETHOD GetForceAllowThirdPartyCookie(PRBool *aForce);
   NS_IMETHOD SetForceAllowThirdPartyCookie(PRBool aForce);
   NS_IMETHOD GetCanceled(PRBool *aCanceled);
+  NS_IMETHOD GetChannelIsForDownload(PRBool *aChannelIsForDownload);
+  NS_IMETHOD SetChannelIsForDownload(PRBool aChannelIsForDownload);
 
   // nsISupportsPriority
   NS_IMETHOD GetPriority(PRInt32 *value);
@@ -230,6 +232,7 @@ protected:
   PRUint32                          mInheritApplicationCache    : 1;
   PRUint32                          mChooseApplicationCache     : 1;
   PRUint32                          mLoadedFromApplicationCache : 1;
+  PRUint32                          mChannelIsForDownload       : 1;
 };
 
 
diff --git a/netwerk/protocol/http/nsHttpChannel.cpp b/netwerk/protocol/http/nsHttpChannel.cpp
index 32a9f17c2415..a0f144298882 100644
--- a/netwerk/protocol/http/nsHttpChannel.cpp
+++ b/netwerk/protocol/http/nsHttpChannel.cpp
@@ -802,6 +802,12 @@ nsHttpChannel::CallOnStartRequest()
     rv = ApplyContentConversions();
     if (NS_FAILED(rv)) return rv;
 
+    // if this channel is for a download, close off access to the cache.
+    if (mCacheEntry && mChannelIsForDownload) {
+        mCacheEntry->Doom();
+        CloseCacheEntry(PR_FALSE);
+    }
+
     if (!mCanceled) {
         // create offline cache entry if offline caching was requested
         if (mCacheForOfflineUse) {
diff --git a/netwerk/protocol/http/nsIHttpChannelInternal.idl b/netwerk/protocol/http/nsIHttpChannelInternal.idl
index e27e37388be0..a10ea3eab34b 100644
--- a/netwerk/protocol/http/nsIHttpChannelInternal.idl
+++ b/netwerk/protocol/http/nsIHttpChannelInternal.idl
@@ -45,7 +45,7 @@ interface nsIProxyInfo;
  * using any feature exposed by this interface, be aware that this interface 
  * will change and you will be broken.  You have been warned.
  */
-[scriptable, uuid(91dbb42a-dffc-4f47-8b27-9579c0d92c3f)]
+[scriptable, uuid(9fb2a161-d075-4bf2-b07a-26bac650cc81)]
 interface nsIHttpChannelInternal : nsISupports
 {
     /**
@@ -88,6 +88,11 @@ interface nsIHttpChannelInternal : nsISupports
      * Returns true iff the channel has been canceled.
      */
     readonly attribute boolean canceled;
+
+    /**
+     * Lets externalhandler tell the channel it is open on behalf of a download
+     */
+    attribute boolean channelIsForDownload;
 };
 
 [uuid(b18290f1-ff34-4e33-93a2-29aeb7b95425)]
diff --git a/uriloader/exthandler/nsExternalHelperAppService.cpp b/uriloader/exthandler/nsExternalHelperAppService.cpp
index 501e4607f600..562534ee3c08 100644
--- a/uriloader/exthandler/nsExternalHelperAppService.cpp
+++ b/uriloader/exthandler/nsExternalHelperAppService.cpp
@@ -94,6 +94,7 @@
 #include "nsMimeTypes.h"
 // used for header disposition information.
 #include "nsIHttpChannel.h"
+#include "nsIHttpChannelInternal.h"
 #include "nsIEncodedChannel.h"
 #include "nsIMultiPartChannel.h"
 #include "nsIFileChannel.h"
@@ -1644,6 +1645,12 @@ NS_IMETHODIMP nsExternalAppHandler::OnStartRequest(nsIRequest *request, nsISuppo
     encChannel->SetApplyConversion( applyConversion );
   }
 
+  // Inform channel it is open on behalf of a download to prevent caching.
+  nsCOMPtr<nsIHttpChannelInternal> httpInternal = do_QueryInterface(aChannel);
+  if (httpInternal) {
+    httpInternal->SetChannelIsForDownload(PR_TRUE);
+  }
+
   // now that the temp file is set up, find out if we need to invoke a dialog
   // asking the user what they want us to do with this content...