Starting with Firefox 56.0.2, ASP Uploader no longer functions properly.  The form shows up, and you can select a file but as soon as you start the upload, the browser logs an error Oxa2 is null.   I have repeated this on 4 seperate computers, and I can verify that it happens on your test page at http://aspuploader.com/demo/form-multiplefiles.asp as well.

Any ideas?

Apparently this is tied not to Firefox directly, but to the Adblock Plus addon (3.0.1) from https://adblockplus.org/ .

If I disable this plug-in then your page(s) work perfectly fine.  Even if I tell it to not block anything on the domain domain, it will not work if the plugin is enabled.  Has anyone experienced this issue with this plugin for Firefox?