diff --git a/uriloader/exthandler/nsExternalHelperAppService.cpp b/uriloader/exthandler/nsExternalHelperAppService.cpp
index d3cadf6b609..67b4373f87e 100644
--- a/uriloader/exthandler/nsExternalHelperAppService.cpp
+++ b/uriloader/exthandler/nsExternalHelperAppService.cpp
@@ -1469,8 +1469,8 @@ nsExternalAppHandler::nsExternalAppHandler(nsIMIMEInfo * aMIMEInfo,
   AppendUTF8toUTF16(aTempFileExtension, mTempFileExtension);
 
   // replace platform specific path separator and illegal characters to avoid any confusion
-  mSuggestedFileName.ReplaceChar(FILE_PATH_SEPARATOR FILE_ILLEGAL_CHARACTERS, '-');
-  mTempFileExtension.ReplaceChar(FILE_PATH_SEPARATOR FILE_ILLEGAL_CHARACTERS, '-');
+  mSuggestedFileName.ReplaceChar(FILE_PATH_SEPARATOR FILE_ILLEGAL_CHARACTERS, '_');
+  mTempFileExtension.ReplaceChar(FILE_PATH_SEPARATOR FILE_ILLEGAL_CHARACTERS, '_');
   
   // Make sure extension is correct.
   EnsureSuggestedFileName();