ProxyBlocker od Nabendu

Blocks requests to any webiste that are in user-defined list.
Visit about:addons, open the ProxyBlocker's preferences, and mention thethe hostnames in the text box, you want to block.

Note that the hostname-matching is very simple: hostnames must exactly match an entry in the list if they are to be blocked. So with the default settings, "example.org" would be blocked but "www.example.org" would be permitted.