diff --git a/uriloader/exthandler/nsExternalHelperAppService.cpp b/uriloader/exthandler/nsExternalHelperAppService.cpp
index adf3313e186..cb5fc87cf4e 100644
--- a/uriloader/exthandler/nsExternalHelperAppService.cpp
+++ b/uriloader/exthandler/nsExternalHelperAppService.cpp
@@ -1154,6 +1154,12 @@ NS_IMETHODIMP nsExternalAppHandler::GetTimeDownloadStarted(PRTime* aTime)
   return NS_OK;
 }
 
+NS_IMETHODIMP nsExternalAppHandler::GetContentLength(PRInt64 *aContentLength)
+{
+  *aContentLength = mContentLength;
+  return NS_OK;
+}
+
 NS_IMETHODIMP nsExternalAppHandler::CloseProgressWindow()
 {
   // release extra state...
diff --git a/uriloader/exthandler/nsIExternalHelperAppService.idl b/uriloader/exthandler/nsIExternalHelperAppService.idl
index 02d3f622b7e..08a7516668f 100644
--- a/uriloader/exthandler/nsIExternalHelperAppService.idl
+++ b/uriloader/exthandler/nsIExternalHelperAppService.idl
@@ -166,4 +166,9 @@ interface nsIHelperAppLauncher : nsICancelable
    * Time when the download started
    */
   readonly attribute PRTime timeDownloadStarted;
+
+  /**
+   * The download content length, or -1 if the length is not available.
+   */
+  readonly attribute PRInt64 contentLength;
 };