if you want a character pointer, you're going to have to |.get()| it

uriloader/exthandler/nsExternalHelperAppService.cpp

@@ -625,7 +625,7 @@ nsresult nsExternalAppHandler::PromptForSaveToFile(nsILocalFile ** aNewFile, con
   nsCOMPtr<nsIHelperAppLauncherDialog> dlgService( do_GetService( NS_IHELPERAPPLAUNCHERDLG_PROGID ) );
   nsresult rv = NS_OK;
   if ( dlgService ) 
-    rv = dlgService->PromptForSaveToFile(mWindowContext, aDefaultFile, NS_ConvertASCIItoUCS2(mTempFileExtension), aNewFile);
+    rv = dlgService->PromptForSaveToFile(mWindowContext, aDefaultFile, NS_ConvertASCIItoUCS2(mTempFileExtension).get(), aNewFile);
 
   return rv;
 }