diff --git a/content/base/public/nsIXMLHttpRequest.idl b/content/base/public/nsIXMLHttpRequest.idl
index 92468271e1d..8ca14616964 100644
--- a/content/base/public/nsIXMLHttpRequest.idl
+++ b/content/base/public/nsIXMLHttpRequest.idl
@@ -224,6 +224,17 @@ interface nsIXMLHttpRequest : nsISupports {
    *   4 COMPLETED     Finished with all operations.
    */
    readonly attribute long readyState;
+
+  /**
+   * Override the mime type returned by the server (if any). This may
+   * be used, for example, to force a stream to be treated and parsed
+   * as text/xml, even if the server does not report it as such. This
+   * must be done before the <code>send</code> method is invoked.
+   *
+   * @param mimetype The type used to override that returned by the server
+   *                 (if any).
+   */
+   void   overrideMimeType(in string mimetype);
 };
 
 
diff --git a/content/base/src/nsXMLHttpRequest.cpp b/content/base/src/nsXMLHttpRequest.cpp
index 8ff1d072a16..90c829d0287 100644
--- a/content/base/src/nsXMLHttpRequest.cpp
+++ b/content/base/src/nsXMLHttpRequest.cpp
@@ -814,6 +814,14 @@ nsXMLHttpRequest::OnStartRequest(nsIRequest *request, nsISupports *ctxt)
   mReadRequest = request;
   mContext = ctxt;
   ChangeState(XML_HTTP_REQUEST_LOADED);
+  if (!mOverrideMimeType.IsEmpty()) {
+    nsresult status;
+    request->GetStatus(&status);
+    nsCOMPtr<nsIChannel> channel = do_QueryInterface(request);
+    if (channel && NS_SUCCEEDED(status)) {
+      channel->SetContentType(mOverrideMimeType.get());
+    }
+  }
   return mXMLParserStreamListener->OnStartRequest(request,ctxt);
 }
 
@@ -1214,6 +1222,15 @@ nsXMLHttpRequest::GetReadyState(PRInt32 *aState)
   return NS_OK;
 }
 
+/* void   overrideMimeType(in string mimetype); */
+NS_IMETHODIMP
+nsXMLHttpRequest::OverrideMimeType(const char* aMimeType)
+{
+  // XXX Should we do some validation here?
+  mOverrideMimeType.Assign(aMimeType);
+  return NS_OK;
+}
+
 // nsIDOMEventListener
 nsresult
 nsXMLHttpRequest::HandleEvent(nsIDOMEvent* aEvent)
diff --git a/content/base/src/nsXMLHttpRequest.h b/content/base/src/nsXMLHttpRequest.h
index d9acd9f6d0c..57d11fe657f 100644
--- a/content/base/src/nsXMLHttpRequest.h
+++ b/content/base/src/nsXMLHttpRequest.h
@@ -161,6 +161,7 @@ protected:
 
   PRInt32 mStatus;
   PRBool mAsync;
+  nsCString mOverrideMimeType;
 };
 
 #endif