--- a/uriloader/exthandler/nsExternalHelperAppService.cpp
+++ b/uriloader/exthandler/nsExternalHelperAppService.cpp
@@ -1564,16 +1564,23 @@ nsExternalAppHandler::MaybeApplyDecoding
   nsCOMPtr<nsIEncodedChannel> encChannel = do_QueryInterface(aRequest);
   if (!encChannel) {
     return;
   }
 
   // Turn off content encoding conversions if needed
   bool applyConversion = true;
 
+  // First, check to see if conversion is already disabled.  If so, we
+  // have nothing to do here.
+  encChannel->GetApplyConversion(&applyConversion);
+  if (!applyConversion) {
+    return;
+  }
+
   nsCOMPtr<nsIURL> sourceURL(do_QueryInterface(mSourceUrl));
   if (sourceURL)
   {
     nsAutoCString extension;
     sourceURL->GetFileExtension(extension);
     if (!extension.IsEmpty())
     {
       nsCOMPtr<nsIUTF8StringEnumerator> encEnum;