You need to add a 'download action'. Firefox adds the setting for specific file types as you come across them - in your other computer, you might not have downloaded a WinZip file before. This is how you do it:
- On the web, find a link to a WinZip file.
- Click on the file link to download it.
- Select how you want Firefox to handle the file:
- Open with: Saves the file to a temporary folder and opens it in the default application for that file type. To select an application, click Choose....
- Save file: Saves the file to the download folder (specified in the Firefox General panel).
- In the Opening file window, check mark Do this automatically for files like this from now on.
Make sure you enable the Do this automatically for files like this from now on option.