It means that the website code - as sent to the browser - contains errors.

This is often because the website has been compressed in some way by a 'proxy server. I've seen this happen on mobile phone connections. The site usually works fine on a non-mobile connection.

That site works fine here.

I found the solution to my own problem by using Internet Explorer instead of Firefox, it turn out that the site i was trying to get into is an old site using proxy servers that Firefox doesn't read properly giving me the XML parsing error:not well-formed. Its not Firefox's fault its just letting you know that this could be a problem site if it lets you in.

I have the same problem, with one of our federal government websites, how do I fix this in FF?!?!

Mike