If Google is translating correctly:

Website says: Could not download the file

Firefox says: Could not be saved because the source file could not be read. Try again later, or contact the server administrator.

I think we need to figure out why the server is showing an error first.

What was the last version of Firefox that could download a folder or multiple files from that page? This most likely requires the server to generate a ZIP file.

Firefox doesn't allow my server to create a zip file. Please fix.

Princeps Tenebrarum said

Firefox doesn't allow my server to create a zip file. Please fix.

I don't understand why the server would be limited by Firefox. Can you submit a request to the vendor to figure out what is causing the problem? They can submit a bug to Mozilla once they spot the issue.

Same here :

https://community.synology.com/enu/forum/7/post/120068?page=3

Maybe try some of these suggestions :

https://windowsreport.com/could-not-be-saved-because-the-source-file-could-not-be-read/

Please see this bug report :

https://bugzilla.mozilla.org/show_bug.cgi?id=1575799

If you want to, you could vote to show your interest for that bug report :

https://bugzilla.mozilla.org/page.cgi?id=voting.html

You can post a comment, only If you have any additional information to add, but keep it to the point and concise.

See the section Commenting, especially #4