As a general rule, running the Web Server and LS Server on different ports on the same host is exposed to cross domain access issues because of the browser-enforced restrictions. See here our findings so far.
In particular, are you using IE older than version 8?

As far as you are only testing, you could setup a work around by setting longer hostnames, like "www.kdes.com" and "push.kdes.com" through the "hosts" file.

The error message on Firefox is new for us and it is unclear whether or not it can be related with your particular scenario. Please keep us informed if you see it again with other hostname configurations.

By the way, what do you mean by ".NET Web Client"? Do you just mean that the web pages are originated from aspx pages?