Hi, after the next time you download a ZIP file, try this:

Open the Downloads list, right-click the file, and choose Always Open Similar Files*

Firefox then should add that file type to the Applications list with a "Use [default application]" action.

You can use the steps in the following article to change it to an Always Ask action if you prefer that: Manage file types and download actions in Firefox.

* If that context menu item is not available, the site might be misidentifying the file as generic binary content instead of identifying it as a specific type. This is harder to work around.

If you still have a link to download a suitable ZIP archive in the history then you can opt to re-download this file to trigger the download panel or search the download history.