Url is "plugins/form/submit"
But this url returns a html page (with the right mimetype)
But some proxies catch the mimetype and re-determine it using the extension

Ex: With Apache, the final page is served with "text/plain" mimetype and so is not correctly rendered on FF

The problem has a workaround changing the mimetype configuration of the proxy, but for simpliest configuration, the url should have ".html" at the end (or may do an external redirection instead of an internal one)