Windows will need the file to have the correct extension type that is the dot and three or four letters at the end of a file (you may need to set Vista to display it - I have not used Vista). That file extension then needs to be associated with the correct program, use the assoc command, or use a right click and open with.

Firefox itself lets you choose what to do with different file types, so you may set it to a particular application, to ask, or to save the file.

• see Applications panel - Set how Firefox handles different types of files
• and http://kb.mozillazine.org/File_types_and_download_actions
• mime types
  techie server centred explanation https://developer.mozilla.org/en/docs/Properly_Configuring_Server_MIME_Types
  simpler general explanation
  http://en.wikipedia.org/wiki/MIME#Introduction
• Troubleshooting What to do if you can't download or save files
  • based on http://kb.mozillazine.org/Unable_to_save_or_download_files

There are also possibilities the file is corrupt or virus checking is interfering so note the additional troubleshooting steps in the links above.

You can try to test the file with the 7-zip program or with another archive manager.

You can get the 7-zip program here: http://www.7-zip.org/
Run the installer and start the 7-zip file manager: 7zFM.exe
Browse to the file and click Test.

It is possible that anti-virus software is corrupting downloaded files or is otherwise interfering with downloading files.
Try to disable the real-time (live) scanning of files in your anti-virus software temporarily to see if that makes downloading work.

See:

• http://kb.mozillazine.org/Unable_to_save_or_download_files
• browser.download.manager.scanWhenDone

Many-Many-Many Thanks. 7-zip solved the problem. The attachment shown as been a zip file can now be opened by 7-zip only. In fact there is a data base inside,readably with Excel or Database Viewer May I understand why ?