Ps : sorry for all the faults (maybe), I'm french.
The problem cause is that Download Express 1.3 does not know about FireFox (this version is very old, older than FireFox.) The recent version should work well.
Thank you very much for your answer ! I understand.
So , maybe I will upgrade my DE... But if it doesn't know firefox at all, it's amazing that the plugin for firefox works, isn't it ? :-) Bah... All I have to do is to create a shortcut to windows/system 32/MetaProducts... I'm joking.
The plugin for FireFox knows about FireFox and Download Express. And it sends to Download Express a command to open the Save As dialog, but the old version of DE doesn't understand this command.